Here is my attempt for the homework for week 1.

Author: Irene-Apex-dev

force-app/main/default/classes/VariablesDataTypesOperators.cls

@@ -18,7 +18,7 @@
  * If statements are not specifically covered in the lesson this module, but review module 2 flow control section for an overview if needed.
  * Resources: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_if_else.htm 
  * 
- * @author Your Name
+ * @author Irene Schild
  */
 
 public with sharing class VariablesDataTypesOperators {
@@ -32,7 +32,12 @@ public with sharing class VariablesDataTypesOperators {
      * @return The sum of the two numbers, or null if either number is null.
      */
     public static Integer addition(Integer a, Integer b) {  
-        return null; // Replace null with the variable you used to store the result
+
+        if (a == null || b == null) {
+            return null;
+        }
+
+        return a + b;
     }
 
     /**
@@ -44,7 +49,12 @@ public with sharing class VariablesDataTypesOperators {
      * @return The difference between the two numbers.
      */
     public static Integer subtraction(Integer a, Integer b) {
-        return null; // Replace null with the variable you used to store the result
+
+        if (a == null || b == null) {
+            return null;
+        }
+
+        return a - b;
     }
 
     /**
@@ -56,7 +66,12 @@ public with sharing class VariablesDataTypesOperators {
      * @return The product of the two numbers.
      */
     public static Integer multiplication(Integer a, Integer b) {
-        return null; // Replace null with the variable you used to store the result
+
+        if (a == null || a == 0 || b == null || b == 0) {
+            return null;
+        }
+
+        return a * b; // Replace null with the variable you used to store the result
     }
 
     /**
@@ -69,7 +84,12 @@ public with sharing class VariablesDataTypesOperators {
      * @return The quotient of the division, or 0 if the denominator is zero.
      */
     public static Double division(Double a, Double b) {
-        return null; // Replace null with the variable you used to store the result
+
+        if (a == null || a == 0 || b == null || b == 0) {
+            return 0;
+        }
+
+        return a / b; // Replace null with the variable you used to store the result
     }
 
     /**
@@ -83,7 +103,8 @@ public with sharing class VariablesDataTypesOperators {
      * @return True if the number is even, False otherwise.
      */
     public static Boolean isEven(Integer num) {
-        return null; // Replace null with the variable you used to store the result
+
+        return Math.mod(num, 2) == 0;
     }
 
     /**
@@ -94,7 +115,7 @@ public with sharing class VariablesDataTypesOperators {
      * @return true if the number is positive, false otherwise.
      */
     public static Boolean isPositive(Integer num) {
-        return null; // Replace null with the variable you used to store the result
+        return num >= 1;
     }
 
     /**
@@ -106,7 +127,7 @@ public with sharing class VariablesDataTypesOperators {
      * @return The concatenated string.
      */
     public static String concatenateStrings(String str1, String str2) {
-        return null; // Replace null with the variable you used to store the result
+        return str1 + str2; 
     }
 
 	/**
@@ -121,7 +142,7 @@ public with sharing class VariablesDataTypesOperators {
 	 * @return The complete sentence as a single String.
 	 */
 	public static String createSentence(String noun, String verb, String endingPunctuation) {
-		return null; // Replace null with the variable you used to store the result
+		return 'The ' + noun + ' is ' + verb + endingPunctuation; 
 	}
 
     /**
@@ -134,7 +155,7 @@ public with sharing class VariablesDataTypesOperators {
      * @return True if the date is in the past, False otherwise.
      */
     public static Boolean isDateInPast(Date dt) {
-        return null; // Replace null with the variable you used to store the result
+        return Date.today() > dt; // Replace null with the variable you used to store the result
     }
 
     /**
@@ -147,7 +168,7 @@ public with sharing class VariablesDataTypesOperators {
      * @return True if the date is today or in the future, False otherwise.
      */
     public static Boolean isDateTodayOrFuture(Date dt) {
-        return null; // Replace null with the variable you used to store the result
+        return Date.today() < dt; 
     }
 
     /**
@@ -159,10 +180,16 @@ public with sharing class VariablesDataTypesOperators {
      * @param minutes The number of minutes.
      * @return The number of milliseconds equivalent to the given number of minutes.
      */
-    Integer MILLISECONDS_PER_MINUTE = null; // Make this value a constant
+    
     public static Integer convertMinutesToMilliseconds(Integer minutes) {
-        Integer milliseconds;
-        return null; // Replace null with the variable you used to store the result
+
+        if (minutes == null) {
+            return null;
+        }
+        Integer MILLISECONDS_PER_MINUTE = 60000; 
+
+        Integer milliseconds = minutes * MILLISECONDS_PER_MINUTE;
+        return milliseconds; 
     }    
 
     /**
@@ -176,7 +203,14 @@ public with sharing class VariablesDataTypesOperators {
      * @return The average of the three numbers.
      */
     public static Double averageOfThreeNumbers(Integer a, Integer b, Integer c) {
-        return null; // Replace null with the variable you used to store the result
+        
+        Double sumOfNumbers = a + b + c;
+
+        if (a == null || b == null || c == null || sumOfNumbers == 0) {
+            return 0;
+        }
+
+        return sumOfNumbers / 3; // Replace null with the variable you used to store the result
     }
 
     /**
@@ -190,11 +224,16 @@ public with sharing class VariablesDataTypesOperators {
      * @return The result of the adjusted arithmetic expression.
      */
     public static Integer adjustOrderOfOperations1(Integer a, Integer b, Integer c) {
+
+        if (c == null) {
+            return null;
+        }
+
         // Add parentheses around the addition operation so that it is performed before multiplication
-        Integer result = a + b * c;
+        Integer result = (a + b) * c;
 
         // Return the result
-        return null; // Replace null with the variable you used to store the result
+        return result; 
     }
 
     /**
@@ -206,8 +245,8 @@ public with sharing class VariablesDataTypesOperators {
         // Add parentheses in the below expression to change the result.
         // The result of the expression as it is right now is 43. 
         // You should add parentheses so that the result of the expression becomes 8.
-        Integer answer = 48 - 15 + 5 * 2;
-        return null; // Replace null with the variable you used to store the result
+        Integer answer = 48 - (15 + 5) * 2;
+        return answer; 
     }
 
     /**
@@ -226,9 +265,14 @@ public with sharing class VariablesDataTypesOperators {
      */
     public static Double complexOrderOfOperations(Integer a, Integer b, Integer c, Integer d, Integer e) {
         // Add parentheses around the multiplication and subtraction operations so that they are performed before division and addition
-        Double result = a * b - c / (Double) d + e;
 
-        return null; // Replace null with the variable you used to store the result
+        if (d == null) {
+            return null;
+        }
+
+        Double result = ((a * b) - c) / (Double) d + e;
+
+        return result; // Replace null with the variable you used to store the result
     }
 
     /**
@@ -242,8 +286,12 @@ public with sharing class VariablesDataTypesOperators {
         final Double SUBTRACT_FACTOR = 32.0;
         final Double MULTIPLY_FACTOR = 5.0;
         final Double DIVIDE_FACTOR = 9.0;
-    
-        return null; // Replace null with the variable you used to store the result
+        
+        if (fahrenheit == null) {
+            return 0;
+        }
+
+        return (fahrenheit - SUBTRACT_FACTOR) * MULTIPLY_FACTOR / DIVIDE_FACTOR;
     }
 
 
@@ -258,12 +306,17 @@ public with sharing class VariablesDataTypesOperators {
      * @return The quotient as a casted (round down) Integer.
      */
     public static Integer performDivisionAndCast(Double a, Double b) {
+
+        if (a == null || a == 0 || b == null || b == 0) {
+            return null;
+        }
+
         // Perform the division and cast (round down) off the result.
-        Double divisionResult = null;
+        Double divisionResult = a / b;
 
         // Write the code for type casting the divisionResult to an Integer
-        Integer roundedResult = null;
-        return null; // Replace null with the variable you used to store the result
+        Integer roundedResult = (Integer) divisionResult;
+        return roundedResult; 
     }
 
     /**
@@ -276,8 +329,12 @@ public with sharing class VariablesDataTypesOperators {
      * @return A Decimal representing the weekly paycheck.
      */
     public static Decimal calculateWeeklyPaycheck(Decimal hourlyRate, Double numberOfHours) {
-        // Calculate the weekly paycheck using the formula: rate multiplied by hours
-        return null; // Replace null with the variable you used to store the result
+        
+        if (hourlyRate == null || hourlyRate == 0 || numberOfHours == null || numberOfHours == 0) {
+            return 0;
+        } 
+
+        return hourlyRate * (decimal) numberOfHours;
     }
 
     /**
@@ -291,7 +348,12 @@ public with sharing class VariablesDataTypesOperators {
      * @return A Decimal representing the monthly paycheck.
      */
     public static Decimal calculateMonthlyPaycheck(Decimal hourlyRate, Double numberOfHours) {
-        return null; // Replace null with the variable you used to store the result
+
+        if (hourlyRate == null || hourlyRate == 0 || numberOfHours == null || numberOfHours == 0) {
+            return 0.0;
+        } 
+
+        return hourlyRate * numberOfHours * 4;
     }
 
     /**
@@ -305,6 +367,11 @@ public with sharing class VariablesDataTypesOperators {
      * @return The total cost after applying the sales tax.
      */
     public static Decimal calculateTotalCost(Decimal pricePerUnit, Integer numberOfUnits, Decimal salesTaxRate) {
-        return null; // Replace null with the variable you used to store the result
+        
+        if (pricePerUnit == null || numberOfUnits == null) {
+            return null;
+        }
+
+        return (pricePerUnit * numberOfUnits) * (1 + salesTaxRate);
     }
 }