At the end of August, Xuan had thrice as many 10-cent coins as 50-cent coins in her piggy bank. She took out 55 10-cent coins and exchanged it for 50-cent coins of the same value. In September, she saved another 5 10-cent coins and some 50-cent coins. She then had twice as many 50-cent coins in the end. She had 120 coins in the end. How many 50-cent coins did she save in September?
| |
10-cent |
50-cent |
Total |
| Before |
3 u |
1 u |
|
| Change 1 |
- 55 |
+ 11 |
|
| Change 2 |
+ 5 |
+ ? |
|
| After |
1 p |
2 p |
120 |
Total number of coins in the end
= 1 p + 2 p
= 3 p
3 p = 120
1 p = 120 ÷ 3 = 40
Number of 10-cent in the end
= 1 p
= 40
Number of 10-cent at first
= 40 - 5 + 55
= 90
3 u = 90
1 u = 90 ÷ 3 = 30
Number of 50-cent coins in the end
= 2 p
= 2 x 40
= 80
Total value of 55 10-cent coins
= 55 x 10
= 550¢
Number of 50-cent coins that was exchanged from 55 10-cent coins
= 550 ÷ 50
= 11
Number of 50-cent coins that Xuan saved in September
= 2 p - 1 u - 11
= 80 - 30 - 11
= 39
Answer(s): 39
