private Decimal getCarAllowances() {
    Decimal value = 0;

    for (Pay_Element__c payElement : payElements) {
      if (payElement.Is_Addition__c && payElement.Car_Allowance__c) {
      value += payElement.Value__c;
      }
    }
    
    return value;
  }

Logical operator can only be applied to Boolean is the error I receive for this method. 

  private Decimal getCarAllowances() {
    Decimal value = 0;

    for (Pay_Element__c payElement : payElements) {
      if (payElement.Is_Addition__c == payElement.Car_Allowance__c) {
      value += payElement.Value__c;
      }
    }
    
    return value;
  }

Comparison arguments must be compatible types: Boolean, Decimal is the error I receive for this one.

 

  private Decimal getCarAllowances() {
    Decimal value = 0;

    for (Pay_Element__c payElement : payElements) {
      if (payElement.Is_Addition__c && payElement.Car_Allowance__c) {
      value += payElement.Value__c;
      }
    }
    
    return value;
  }

Logical operator can only be applied to Boolean is the error I receive for this method. 

  private Decimal getCarAllowances() {
    Decimal value = 0;

    for (Pay_Element__c payElement : payElements) {
      if (payElement.Is_Addition__c == payElement.Car_Allowance__c) {
      value += payElement.Value__c;
      }
    }
    
    return value;
  }

Comparison arguments must be compatible types: Boolean, Decimal is the error I receive for this one.