At the end of February, Erika 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 March, she saved another 36 10-cent coins and some 20-cent coins. She then had twice as many 20-cent coins in the end. She had 210 coins in the end. How many 20-cent coins did she save in March?

	10-cent	20-cent	Total
Before	2 u	1 u
Change 1	- 50	+ 25
Change 2	+ 36	+ ?
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 10-cent in the end
= 1 p
= 70

Number of 10-cent at first
= 70 - 36 + 50
= 84

2 u = 84

1 u = 84 ÷ 2 = 42

Number of 20-cent coins in the end
= 2 p
= 2 x 70
= 140

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 Erika saved in March
= 2 p - 1 u - 25
= 140 - 42 - 25 
= 73 

Answer(s): 73