John has a total of 20 fifty-cent and twenty-cent coins. The total value of the coins is $7. If he spends all the fifty-cent coins on a pen, how much does the pen cost?
Number of
fifty-cent coins |
Value of
fifty-cent coins |
Number of
twenty-cent coins |
Value of
twenty-cent coins |
Total
value |
| 20 |
20 x 50
= 1000¢ |
0 |
0 |
1000¢ |
| 19 |
19 x 50
= 950¢ |
1 |
1 x 20
= 20¢ |
970¢ |
| 10 |
10 x 50
= 500¢ |
10 |
10 x 20
= 200¢ |
700¢ |
$1 = 100¢
$7 = 700¢
Total value if all the coins are fifty-cent coins
= 20 x 50
= 1000¢
Big difference between the value of fifty-cent coins and twenty-cent coins
= 1000 - 700
= 300¢
Small difference between the value of one fifty-cent coin and one twenty-cent coin
= 50 - 20
= 30¢
Number of twenty-cent coins
= 300 ÷ 30
= 10
Number of fifty-cent coins
= 20 - 10
= 10
Cost of the pen
= 10 x 50
= 500¢
= $5
Answer(s): $5
