Why does this work perfect in xCode but not here?

Because your code working and actually doing what the challenge asks for are not the same thing. The challenge asks you to add (not set) the incrementing value to the mathTotal variable inside the loop, you are not adding the value to it. Instead you are setting the variable equal to the value.

In other words after the first loop mathTotal should equal 5, after the second loop it should equal 11 (5+6) then 29 (11+7) and so on. With your loop it equals 5 in the first loop, 6 in the second and 7 in the third. As you can see your code is not achieving the desired effect.

You can fix your code by simply changing the = operator to += like this:

int mathTotal;

bool isComplete;

for (int i=5; i <=25; i++) {
mathTotal += i;
}
isComplete = TRUE;

+= adds the value on the right to the variable on the left, instead of simply replacing it entirely like the = operator does.

Thanks for the correct answer. Don't recall being introduced to the += .

Again Thanks.