Moved string-to-integer conversion

This will simplify the student task if we give them partial code to be
completed.

MutualRecursion.py

@@ -77,19 +77,19 @@ def parse(expression):
 
 
 def add(left_expression, right_expression):
-    return int(evaluate(left_expression)) + int(evaluate(right_expression))
+    return evaluate(left_expression) + evaluate(right_expression)
 
 
 def subtract(left_expression, right_expression):
-    return int(evaluate(left_expression)) - int(evaluate(right_expression))
+    return evaluate(left_expression) - evaluate(right_expression)
 
 
 def multiply(left_expression, right_expression):
-    return int(evaluate(left_expression)) * int(evaluate(right_expression))
+    return evaluate(left_expression) * evaluate(right_expression)
 
 
 def divide(left_expression, right_expression):
-    return int(evaluate(left_expression)) / int(evaluate(right_expression))
+    return evaluate(left_expression) / evaluate(right_expression)
 
 
 operator_map = {
@@ -102,7 +102,7 @@ operator_map = {
 
 def evaluate(expression):
     if expression.isnumeric():
-        return expression
+        return int(expression)
     else:
         left_expression, operator, right_expression = parse(expression)
         return operator_map[operator](left_expression, right_expression)