interest-is-interesting: remove negative balance (#1695)

Author: James-N-M

exercises/concept/interest-is-interesting/.docs/hints.md

@@ -10,7 +10,7 @@
 
 ## 2. Calculate the interest
 
-- When calculating interest, it might be helpful to convert a negative balance to a positive one. One could use arithmetic here, or one of the methods in the [`Math` class][docs-microsoft.com-system.math].
+- When calculating interest, it might be helpful to notice that `InterestRate` returns a percentage.
 
 ## 3. Calculate the annual balance update
 

exercises/concept/interest-is-interesting/.docs/instructions.md

@@ -2,7 +2,7 @@
 
 In this exercise you'll be working with savings accounts. Each year, the balance of your savings account is updated based on its interest rate. The interest rate your bank gives you depends on the amount of money in your account (its balance):
 
-- -3.213% for a negative balance.
+- 3.213% for a negative balance (balance gets more negative).
 - 0.5% for a positive balance less than `1000` dollars.
 - 1.621% for a positive balance greater or equal than `1000` dollars and less than `5000` dollars.
 - 2.475% for a positive balance greater or equal than `5000` dollars.

exercises/concept/interest-is-interesting/.meta/Exemplar.cs

@@ -6,7 +6,7 @@ public static float InterestRate(decimal balance)
     {
         if (balance < 0.0m)
         {
-            return -3.213f;
+            return 3.213f;
         }
 
         if (balance < 1000.0m)
@@ -24,7 +24,7 @@ public static float InterestRate(decimal balance)
 
     public static decimal Interest(decimal balance)
     {
-        return Math.Abs(balance) * ((decimal)InterestRate(balance) / 100);
+        return balance * ((decimal)InterestRate(balance) / 100);
     }
 
     public static decimal AnnualBalanceUpdate(decimal balance)

exercises/concept/interest-is-interesting/InterestIsInterestingTests.cs

@@ -68,30 +68,30 @@ public void Large_third_interest_rate()
 
     [Fact]
     [Task(1)]
-    public void Minimal_negative_interest_rate()
+    public void Rate_on_minimal_negative_balance()
     {
-        Assert.Equal(-3.213f, SavingsAccount.InterestRate(-0.000001m));
+        Assert.Equal(3.213f, SavingsAccount.InterestRate(-0.000001m));
     }
 
     [Fact]
     [Task(1)]
-    public void Small_negative_interest_rate()
+    public void Rate_on_small_negative_balance()
     {
-        Assert.Equal(-3.213f, SavingsAccount.InterestRate(-0.123m));
+        Assert.Equal(3.213f, SavingsAccount.InterestRate(-0.123m));
     }
 
     [Fact]
     [Task(1)]
-    public void Regular_negative_interest_rate()
+    public void Rate_on_regular_negative_balance()
     {
-        Assert.Equal(-3.213f, SavingsAccount.InterestRate(-300.0m));
+        Assert.Equal(3.213f, SavingsAccount.InterestRate(-300.0m));
     }
 
     [Fact]
     [Task(1)]
-    public void Large_negative_interest_rate()
+    public void Rate_on_large_negative_balance()
     {
-        Assert.Equal(-3.213f, SavingsAccount.InterestRate(-152964.231m));
+        Assert.Equal(3.213f, SavingsAccount.InterestRate(-152964.231m));
     }
 
     [Fact]