A coin bank contains some ten-cent and twenty-cent coins in the ratio of 5 : 7.
When 14 twenty-cent coins are taken out and
exchanged with some ten-cent coins of the same value,
the ratio became 6 : 7.
Find the sum of money in the coin box in dollars.

	Make p the same. (1) x 7 = (3)	(1) 10-cent	(2) 20-cent	Make p the same. (2) x 6 = (4)
Before	35 u	5 u	7 u	42 u
Change	+ 196	+ 28	- 14	- 84
After	42 p	6 p	7 p	42 p

Value of 14 twenty-cent coins
= 14 x 20
= 280¢

Number of ten-cent coins to exchange for
= 280 ÷ 10
= 28

5 u + 28 = 6 p --- (1)
7 u - 14 = 7 p --- (2)

Make p the same.

(1) _{x 7}
35 u + 196 = 42 p --- (3)

(2) _{x 6}
42 u - 84 = 42 p --- (4)

(4) = (3)
42 u - 84 = 35 u + 196
42 u - 35 u = 196 + 84
7 u = 280

1 u = 280 ÷ 7 = 40

Value of ten-cent coins
= 5 u x 10
= 5 x 40 x 10
= 2000¢

Value of twenty-cent coins
= 7 u x 20
= 7 x 40 x 20
= 5600¢

Total value of coins
= 2000 + 5600
= 7600¢

100¢ = $1

Sum of money in the coin box
= 7600¢
= $76

Answer(s): $76