Risa bought 25% more erasers than Oliver. Sofia bought 20% fewer erasers than Risa. Risa and Oliver gave Sofia some erasers in the ratio 3 : 2. As a result, Sofia had twice as much erasers as before. Given that Oliver had 39 more erasers than Risa in the end, how much erasers did Sofia receive?

	Risa	Oliver	Sofia
	5x5	4x5
	5x5		4x5
Before	25 u	20 u	20 u
Change			+ (2 x 20 u) = + 40 u
	- 3x8 = - 24 u	- 2x8 = - 16 u	+ 5x8 =  + 40 u
After	1 u	4 u	60 u

100% + 25% = 125%
125% = ¹²⁵₁₀₀ = ⁵₄

100% - 20% = 80%
80% = ⁸⁰₁₀₀ = ⁴₅

The number of erasers that Risa had at first is the repeated identity.
LCM of 5 and 5 = 25

The change in the number of erasers that Sofia had is the repeated identity when Risa and Oliver gave to Sofia.
LCM of 40 and 5 = 40

Number of erasers that Oliver had more than Risa in the end
= 4 u - 1 u
= 3 u

3 u = 39
1 u = 39 ÷ 3 = 13

Number of erasers that Sofia received
= 24 u + 16 u
= 40 u
= 40 x 13
= 520

Answer(s): 520