Liam has 9/10 of a gallon of paint, this quantity is the same as 18/20, that is,
9/10 = 18/20
9 and 10 are multiply by 2 to get 18 and 20 so that the fraction of paint available will have the same denominator as the quantity of paint that each bird house required.
The quantity of paint available is 18/20 and each house bird requires a quantity of 1/20 of gallon of paint, this means that 18 bird houses can be painted with the quantity of paint available.