Percent problems confuse lots of people. When you see the question, "What is 35% of 40?" Try to use critical thinking and ask yourself, "What is half , or 50% of 40?" Most people can answer that with mental math. The answer to half of 40 would be 20. Therefore, you know that your answer has to be less than 20.
Estimating and rounding can be very helpful with math in general. Also, when you see the word "of" in a percent problem it means to multiply.
35% of 40 is the same as 35/100 (or .35) X 40 = 14
I recommended rounding and estimating to make doing math in your head easier. That way you have a way of getting an idea of what your answer should look like. This will keep you on track better than just computing numbers blindly or just using a calculator.