At the end of January, Lynn had thrice as many 20-cent coins as 50-cent coins in her piggy bank. She took out 85 20-cent coins and exchanged it for 50-cent coins of the same value. In February, she saved another 53 20-cent coins and some 50-cent coins. She then had twice as many 50-cent coins in the end. She had 210 coins in the end. How many 50-cent coins did she save in February?

	20-cent	50-cent	Total
Before	3 u	1 u
Change 1	- 85	+ 34
Change 2	+ 53	+ ?
After	1 p	2 p	210

Total number of coins in the end
= 1 p + 2 p 
= 3 p

3 p = 210

1 p = 210 ÷ 3 = 70

Number of 20-cent in the end
= 1 p
= 70

Number of 20-cent at first
= 70 - 53 + 85
= 102

3 u = 102

1 u = 102 ÷ 3 = 34

Number of 50-cent coins in the end
= 2 p
= 2 x 70
= 140

Total value of 85 20-cent coins 
= 85 x 20
= 1700¢

Number of 50-cent coins that was exchanged from 85 20-cent coins 
= 1700 ÷ 50
= 34

Number of 50-cent coins that Lynn saved in February
= 2 p - 1 u - 34
= 140 - 34 - 34 
= 72 

Answer(s): 72