A coin bank contains some five-cent and twenty-cent coins in the ratio of 3 : 4.
When 20 twenty-cent coins are taken out and
exchanged with some five-cent coins of the same value,
the ratio became 10 : 7.
Find the sum of money in the coin box in dollars.

	Make p the same. (1) x 7 = (3)	(1) 5-cent	(2) 20-cent	Make p the same. (2) x 10 = (4)
Before	21 u	3 u	4 u	40 u
Change	+ 560	+ 80	- 20	- 200
After	70 p	10 p	7 p	70 p

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

Number of five-cent coins to exchange for
= 400 ÷ 5
= 80

3 u + 80 = 10 p --- (1)
4 u - 20 = 7 p --- (2)

Make p the same.

(1) _{x 7}
21 u + 560 = 70 p --- (3)

(2) _{x 10}
40 u - 200 = 70 p --- (4)

(4) = (3)
40 u - 200 = 21 u + 560
40 u - 21 u = 560 + 200
19 u = 760

1 u = 760 ÷ 19 = 40

Value of five-cent coins
= 3 u x 5
= 3 x 40 x 5
= 600¢

Value of twenty-cent coins
= 4 u x 20
= 4 x 40 x 20
= 3200¢

Total value of coins
= 600 + 3200
= 3800¢

100¢ = $1

Sum of money in the coin box
= 3800¢
= $38

Answer(s): $38