If you just want those three questions, I would write 3 numeric questions and skip calculated. If you want to ask someone any combination of additions of numbers divisible by 5, I would write it this way.
Use the simplified calculated for addition because the calculated version chokes when the tolerance is 0 and there is no way to make it through writing the question.
What is the sum of {{x}*5} and {{y*5}?
Where the variables are {x} and {y} and the decimal places are 0 and the tolerance is 0. I would suggest the range for each be 1 to 100, but that would be up to you. Add at least 10 sets.
The answer is ({x}+{y})*5