Verified Answer :

Curti

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

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

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