A coin bank contains some ten-cent and twenty-cent coins in the ratio of 4 : 5.
When 20 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 |
28 u |
4 u |
5 u |
30 u |
Change |
+ 280 |
+ 40 |
- 20 |
- 120 |
After |
42 p |
6 p |
7 p |
42 p |
Value of 20 twenty-cent coins
= 20 x 20
= 400¢
Number of ten-cent coins to exchange for
= 400 ÷ 10
= 40
4 u + 40 = 6 p --- (1)
5 u - 20 = 7 p --- (2)
Make p the same.
(1)
x 7 28 u + 280 = 42 p --- (3)
(2)
x 6 30 u - 120 = 42 p --- (4)
(4) = (3)
30 u - 120 = 28 u + 280
30 u - 28 u = 280 + 120
2 u = 400
1 u = 400 ÷ 2 = 200
Value of ten-cent coins
= 4 u x 10
= 4 x 200 x 10
= 8000¢
Value of twenty-cent coins
= 5 u x 20
= 5 x 200 x 20
= 20000¢
Total value of coins
= 8000 + 20000
= 28000¢
100¢ = $1
Sum of money in the coin box
= 28000¢
= $280
Answer(s): $280