3) Three students in the class received an A on a test.The three students represent 15 percent of the class.How many students are in the class?

If the mental math isn't intuitive for you, there's an easy way to remember how to do this. Normally, if you're given a number and have to find a certain percentage of that number, you multiply. For example 35% of 20 would be 20 x .35 = 7. If you're told
that the number you're given is a certain percentage of another number and you have to find that number, you're basically doing the previous problem in reverse, so instead of multiplying, you do the opposite operation: divide! 7 ÷ .35 = 20

So for your problems:

15 ÷ .12 = 125

36 ÷ 2.00 = 18

3 ÷ .15 = 20