Question

A city has 3 new houses for every 9 old houses. If there are 15 new houses in the city, how many old houses are there?

Answer 1

45 old houses

Explanation:

What I did:

I divided 15 by 3, to get an idea of the 'scale' which gives us 5

for every 9 old houses, we have 3 new ones; so to scale be multiply the base number by our "scale" number, 5.

3 * 5 = 15 new houses

8 * 5 = 45 old houses

Answer 2

45 old houses

Explanation:

new houses : old houses

3 : 9

15 : ?

(3 = 1st ratio and 9 = 2nd ratio)

To find the missing ratio, divide the first ratio from the new ratio:

15 / 3 = 5

Take the product and multiply that by the second ratio to find the missing ratio:

9 x 5 = 45