At the end of November, Anna had four times as many 10-cent coins as 20-cent coins in her piggy bank. She took out 100 10-cent coins and exchanged it for 20-cent coins of the same value. In December, 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 180 coins in the end. How many 20-cent coins did she save in December?
|
10-cent |
20-cent |
Total |
Before |
4 u |
1 u |
|
Change 1 |
- 100 |
+ 50 |
|
Change 2 |
+ 8 |
+ ? |
|
After |
1 p |
2 p |
180 |
Total number of coins in the end
= 1 p + 2 p
= 3 p
3 p = 180
1 p = 180 ÷ 3 = 60
Number of 10-cent in the end
= 1 p
= 60
Number of 10-cent at first
= 60 - 8 + 100
= 152
4 u = 152
1 u = 152 ÷ 4 = 38
Number of 20-cent coins in the end
= 2 p
= 2 x 60
= 120
Total value of 100 10-cent coins
= 100 x 10
= 1000¢
Number of 20-cent coins that was exchanged from 100 10-cent coins
= 1000 ÷ 20
= 50
Number of 20-cent coins that Anna saved in December
= 2 p - 1 u - 50
= 120 - 38 - 50
= 32
Answer(s): 32