A coin bank contains some twenty-cent and fifty-cent coins in the ratio of 3 : 4. When 12 fifty-cent coins were taken out and replaced by some twenty-cent coins, the ratio became 7 : 6. The total value of the twenty-cent coins added was the same as the total value of the fifty-cent coins taken out. Find the sum of money in the coin box.

	Make p the same (1)x6 = (3)	20-cent (1)	50-cent (2)	Make p the same (2)x7 = (4)
Before	18 u	3 u	4 u	28 u
Change	+ 180	+ 30	- 12	- 84
After	42 p	7 p	6 p	42 p

Value of 50-cent coins taken out
= 12 x 0.50
= $6

Number of 20-cent coins put in
= 6 ÷ 0.20
= 30

(3) = (4)
28 u - 84 = 18 u + 180
28 u - 18 u = 180 + 84
12 u = 264

1 u = 264 ÷ 12 = 22

Number of 20-cent coins at first
= 3 u
= 3 x 22
= 66

Number of 50-cent coins at first
= 4 u
= 4 x 22
= 88

Sum of money in the coin box
= 66 x 0.20 + 88 x 0.50
= 13.20 + 44.00
= $57.20

Answer(s): $57.20