Andy has 9 more 20-cent coins than 10-cent coins. After he used 15 of his 20-cent coins, the total value of 20-cent coins is $3.70 more than the total value of 10-cent coins. How many coins did he have at first?

	20¢	10¢
Before	1 u + 9	1 u
Change	- 15
After	1 u - 6	1 u

	20¢	10¢
Number	1 u - 6	1 u
Value	20	10
Total value	20 u - 120	10 u

$1 = 100¢
$3.70 = 370¢

Number of 20¢ coins in the end
= 1 u + 9 - 15
= 1 u - 6

Total value of 20¢ coins in the end
= 20 x (1 u - 6)
= 20 u - 120

Total value of 10¢ coins in the end
= 10 x 1 u
= 10 u

After using 15 20-cent coins, the value of 20-cent coins is 370¢ more than the value of 10-cent coins. If another 370¢ is added to the 10-cent coins, the total value of 20-cent coins and 10-cent coins will be the same.

20 u - 120 = 10 u + 370
20 u - 10 u = 370 + 120
10 u = 490

1 u = 490 ÷ 10 = 49

Number of coins that Andy had at first
= 1 u + 1 u + 9
= 2 u + 9
= (2 x 49) + 9
= 98 + 9
= 107

Answer(s): 107