Crate C contains 2 grey marbles and 9 yellow marbles. Crate D contains 87 grey marbles and 23 yellow marbles. How many yellow marbles and grey marbles must be transferred from Crate D to put into Crate C so that 50% of the marbles in Crate A are grey and 80% of the marbles in Crate D are grey?

	Crate C		Crate D
	Grey marbles	Yellow marbles	Grey marbles	Yellow marbles
Before	2	9	87	23
Change	+ ?	+ ?	- ?	- ?
After	1 u	1 u	4 p	1 p

50% = ⁵⁰₁₀₀ = 12

80% = ⁸⁰₁₀₀ = 45

Number of grey marbles = 2 + 87 = 89 

Number of yellow marbles = 9 + 23 = 32 

1 u + 4 p = 89 --- (1)

1 u + 1 p = 32 ---(2)

(1) - (2)
(1 u + 4 p) - (1 u + 1 p) = 89 - 32
4 p - 1 p = 57
3 p = 57

1 p = 57 ÷ 3 = 19

From (2):
1 u + 1 p = 32
1 u + 1 x 19 = 32
1 u + 19 = 32

1 u = 32 - 19 = 13

Number of yellow marbles to be transferred from Crate D to Crate C
= 23 - 1 p
= 23 - 1 x 19
= 23 - 19
= 4

Number of grey marbles to be transferred from Crate D to Crate C
= 1 u - 2
= 13 - 2
= 11

Total number of yellow and grey marbles to be transferred from Crate D to Crate C
= 4 + 11
= 15

Answer(s): 15