So this is called division. To solve this you can either use a calculator, or you can use something called guess and check. Guess and check is what it sounds like, guessing and checking to see if it's correct or not.
The way I would solve it is just do 100/20 which equals 5. You can also just add 20 a certain amount of times. For example you could do 20 + 20 + 20 + 20 which equals 80. So that means you can add another 20 to equal 100
