If you want the ratio to be 100 : ?, then you have to multiply both sides of the 20 : 5 ratio to make it that. To find what to multiply both sides by, divide 100 by 20, to get 5.
Now, multiply both sides of the ratio:
20 * 5 : 5 * 5 = 100 : 25
Therefore, if there are 100 golf balls in the bag, there are 25 tennis balls.
