4in 6 houses have a dog. there are 12 houses. how many have a dog?

a: 6
b: 4
c: 8

If 4 in six houses have a dog you can make that a fraction like 4/6.
The question is asking to have a 12 at the bottom of the fraction like x/12
to get to 12 from 6 you multiplied by 2. 
Multiply 4 by 2 and that is your answer.