At the end of June, Elyse had thrice as many 10-cent coins as 20-cent coins in her piggy bank. She took out 70 10-cent coins and exchanged it for 20-cent coins of the same value. In July, she saved another 31 10-cent coins and some 20-cent coins. She then had four times as many 20-cent coins in the end. She had 450 coins in the end. How many 20-cent coins did she save in July?

	10-cent	20-cent	Total
Before	3 u	1 u
Change 1	- 70	+ 35
Change 2	+ 31	+ ?
After	1 p	4 p	450

Total number of coins in the end
= 1 p + 4 p 
= 5 p

5 p = 450

1 p = 450 ÷ 5 = 90

Number of 10-cent in the end
= 1 p
= 90

Number of 10-cent at first
= 90 - 31 + 70
= 129

3 u = 129

1 u = 129 ÷ 3 = 43

Number of 20-cent coins in the end
= 4 p
= 4 x 90
= 360

Total value of 70 10-cent coins 
= 70 x 10
= 700¢

Number of 20-cent coins that was exchanged from 70 10-cent coins 
= 700 ÷ 20
= 35

Number of 20-cent coins that Elyse saved in July
= 4 p - 1 u - 35
= 360 - 43 - 35 
= 282 

Answer(s): 282