A household of three humans has cats and dogs in the ratio 1 : 2. If the total number of legs in the household is 42, what is the total number of cats in the household?

3

3 humans,

Cats : Dogs = 1 : 2

⇒ Humans have 2 legs, cats and dogs have 4 legs each, therefore,

⇒ 3(2) + a(4) + 2a(4) = 42

⇒ 6 + 4a + 8a = 42

⇒ 12a = 42 - 6

⇒ 12a = 36

⇒ a = 3,

Therefore there are = 1 x 3 = 3 cats.