In this question, each birdhouse requires 1/20 gallon of paint. From this description you can get this equation:
1 birdhouse= 1/20 gallon of paint
Then 9/10 gallon of paint would equal to:
9/10 gallon of paint / (1/20 gallon of paint/1 birdhouse)=
9/10 gallon of paint *20 birdhouse / 1 gallon of paint=
180/10 * birdhouse= 18 birdhouse