What is the ratio equivalent of 15/20?

A ratio stays equivalent when you multiply or divide the same thing to both numbers, just like fractions:

First of all, you can simplify it to

3:4 by dividing both numbers by 5.

Now you can multiply both numbers by 5 to get 75:100

Finally, you can multiply both numbers by 3 to get the ratio 45:60

15/20 simplifies to 3/4