At the end of June, Gwen had thrice as many 10-cent coins as 50-cent coins in her piggy bank. She took out 75 10-cent coins and exchanged it for 50-cent coins of the same value. In July, she saved another 16 10-cent coins and some 50-cent coins. She then had thrice as many 50-cent coins in the end. She had 280 coins in the end. How many 50-cent coins did she save in July?

	10-cent	50-cent	Total
Before	3 u	1 u
Change 1	- 75	+ 15
Change 2	+ 16	+ ?
After	1 p	3 p	280

Total number of coins in the end
= 1 p + 3 p 
= 4 p

4 p = 280

1 p = 280 ÷ 4 = 70

Number of 10-cent in the end
= 1 p
= 70

Number of 10-cent at first
= 70 - 16 + 75
= 129

3 u = 129

1 u = 129 ÷ 3 = 43

Number of 50-cent coins in the end
= 3 p
= 3 x 70
= 210

Total value of 75 10-cent coins 
= 75 x 10
= 750¢

Number of 50-cent coins that was exchanged from 75 10-cent coins 
= 750 ÷ 50
= 15

Number of 50-cent coins that Gwen saved in July
= 3 p - 1 u - 15
= 210 - 43 - 15 
= 152 

Answer(s): 152