The pet store has 6 / 2 = 3 cats for sale. They have 6 x 2 = 12 birds for sale. They have 6 x 3 = 18 birds for sale. Therefore, there are a total of 6 + 3 + 12 + 18 = 39 pets for sale.
This justifies what answer for what question? Q & A:
A pet store has six dogs for sale. They have half as many cats, twice as many birds, and thrice as many fish for sale. How many animals are for sale in the pet store?
39