"The ratio of the number of cats to dogs is 3:5. How many dogs would there be if there were 12 cats?"

Reworded, this says that "for every 3 cats there are 5 dogs." Or else you could say, "for every 5 dogs there are 3 cats."

You are given the number of cats=12. For every 3 cats there are 5 dogs." That means 50 dogs.

(3 cats) / (5 dogs) = (12 cats) / (x dogs)

3 / 5 = 12 / x

3x = 5*12

3x = 60

x = 20 dogs

Or you could write:

(5 dogs) / (3 cats) = (x dogs) / (12 cats)

x = 20 dogs.

Or you could visualize this:

CCC:DDDD

[12 cats] CCC CCC CCC CCC : DDDDD DDDDD DDDDD DDDDD [20 Dogs]