One way of approaching this would be to add up 3 and 2, obtaining 5, and then multiplying 15 by 3/5:
15(3/5) = 9
Then write and solve the equation of ratios:
3 9
-- = ------
2 x
Then 3x=18, and x=6. Thus, 15 divides into 9 and 6, which has the ratio 3:2.
