The number of five-cent coins in a coin box was ¹₃ the number of twenty-cent coins. Tom took out 15 twenty-cent coins and exchanged them for five-cent coins. Then he put the money back into the coin box. The number of twenty-cent coins became ²₅ the number of five-cent coins. How much money was there in the coin box?

	Make u the same (1)x3 = (3)	5-cent (1)	20-cent (2)
Before	3 u	1 u	3 u
Change	+ 180	+ 60	- 15
After	15 p	5 p	2 p

Value of 20-cent coins to be exchanged to 5-cent coins
= 15 x 20
= 300

Number of 5-cent coins to be exchanged from the 20-cent coins
= 300 ÷ 5
= 60

(3) = (2)
15 p - 180 = 2 p + 15
15 p - 2 p = 15 + 180
13 p = 195

1 p = 195 ÷ 13 = 15

Number of 5-cent coins in the end
= 5 p
= 5 p x 15
= 75

Value of 5-cent coins in the end
= 75 x 5
= 375¢

Number of 20-cent coins in the end
= 2 p
= 2 x 15
= 30

Value of 20-cent coins in the end
= 30 x 20
= 600¢

Total value of coins
= 375 + 600
= 975¢

$1 = 100¢
975¢ = $9.75 

Answer(s): $9.75