Question

Liam has 9/10 gallon of paint for painting the birdhouses he sells at the

Answer 1

We know two things: The first is that Liam has 9/10 gallon of paint and the second is that each birdhouse requires 1/20 gallon of paint.
In order to calculate how many birdhouse Lian can paint with 9/10 gallon of paint we should divide 9/10 with 1/20:
(9/10) / (1/20)= 9*20/10=180/10=18
So, Liam can paint 18 birdhouses.

Answer 2

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.