Bobby, Riordan and Oscar have some erasers. The ratios of their erasers are as follows:

Bobby : (Bobby + Riordan + Oscar) = 1 : 5
Riordan : (Bobby + Oscar) = 1 : 3

Oscar has 42 erasers more than Riordan. In the end, Oscar gives away all he has to Riordan and Bobby so that both Riordan and Bobby end up with the same number of erasers, how many erasers has Riordan received from Oscar?

Riordan	Oscar	Bobby	Total
4x4		1x4	5x4
1x5	3x5		4x5
5 u	11 u	4 u	20 u

The total number of erasers is repeated.  Make the total number of erasers the same.  LCM of 5 and 4 is 20.

Number of erasers that Oscar has more than Riordan
= 11 u - 5 u
= 6 u

6 u = 42

1 u = 42 ÷ 6 = 7

	Riordan	Oscar	Bobby
Before	5 u	11 u	4 u
Change	+ 5 u	- 11 u	+ 6 u
After	10 u	0 u	10 u

Number of erasers that Riordan has received from Oscar
= 10 u - 5 u 
= 5 u
= 5 x 7
= 35

Answer(s): 35