U have to think what numbers go into both those numbers the biggest number that will go in 9 goes into 90 and 18 it goes to 90 ten times and into 18 twice so the ratio in its simplest form it 10:2
To find the ratio between 2 numbers, you need to find the GCF of those 2 numbers. If the GCF is the ratio, the ratio is in it's simplest form. GCF of 90 and 18 is 18 divide both numbers by the GCF and the final simplest ratio is 5:1
