At the end of April, Yen had twice as many 10-cent coins as 20-cent coins in her piggy bank. She took out 50 10-cent coins and exchanged it for 20-cent coins of the same value. In May, she saved another 8 10-cent coins and some 20-cent coins. She then had twice as many 20-cent coins in the end. She had 120 coins in the end. How many 20-cent coins did she save in May?
| |
10-cent |
20-cent |
Total |
| Before |
2 u |
1 u |
|
| Change 1 |
- 50 |
+ 25 |
|
| Change 2 |
+ 8 |
+ ? |
|
| 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 - 8 + 50
= 82
2 u = 82
1 u = 82 ÷ 2 = 41
Number of 20-cent coins in the end
= 2 p
= 2 x 40
= 80
Total value of 50 10-cent coins
= 50 x 10
= 500¢
Number of 20-cent coins that was exchanged from 50 10-cent coins
= 500 ÷ 20
= 25
Number of 20-cent coins that Yen saved in May
= 2 p - 1 u - 25
= 80 - 41 - 25
= 14
Answer(s): 14
