A coin bank contains some ten-cent and twenty-cent coins in the ratio of 3 : 7.
When 20 twenty-cent coins are taken out and
exchanged with some ten-cent coins of the same value,
the ratio became 7 : 5.
Find the sum of money in the coin box in dollars.

	Make p the same. (1) x 5 = (3)	(1) 10-cent	(2) 20-cent	Make p the same. (2) x 7 = (4)
Before	15 u	3 u	7 u	49 u
Change	+ 200	+ 40	- 20	- 140
After	35 p	7 p	5 p	35 p

Value of 20 twenty-cent coins
= 20 x 20
= 400¢

Number of ten-cent coins to exchange for
= 400 ÷ 10
= 40

3 u + 40 = 7 p --- (1)
7 u - 20 = 5 p --- (2)

Make p the same.

(1) _{x 5}
15 u + 200 = 35 p --- (3)

(2) _{x 7}
49 u - 140 = 35 p --- (4)

(4) = (3)
49 u - 140 = 15 u + 200
49 u - 15 u = 200 + 140
34 u = 340

1 u = 340 ÷ 34 = 10

Value of ten-cent coins
= 3 u x 10
= 3 x 10 x 10
= 300¢

Value of twenty-cent coins
= 7 u x 20
= 7 x 10 x 20
= 1400¢

Total value of coins
= 300 + 1400
= 1700¢

100¢ = $1

Sum of money in the coin box
= 1700¢
= $17

Answer(s): $17