Crate C contains 7 green marbles and 6 grey marbles. Crate D contains 27 green marbles and 13 grey marbles. How many grey marbles and green marbles must be transferred from Crate D to put into Crate C so that 50% of the marbles in Crate A are green and 80% of the marbles in Crate D are green?

	Crate C		Crate D
	Green marbles	Grey marbles	Green marbles	Grey marbles
Before	7	6	27	13
Change	+ ?	+ ?	- ?	- ?
After	1 u	1 u	4 p	1 p

50% = ⁵⁰₁₀₀ = 12

80% = ⁸⁰₁₀₀ = 45

Number of green marbles = 7 + 27 = 34 

Number of grey marbles = 6 + 13 = 19 

1 u + 4 p = 34 --- (1)

1 u + 1 p = 19 ---(2)

(1) - (2)
(1 u + 4 p) - (1 u + 1 p) = 34 - 19
4 p - 1 p = 15
3 p = 15

1 p = 15 ÷ 3 = 5

From (2):
1 u + 1 p = 19
1 u + 1 x 5 = 19
1 u + 5 = 19

1 u = 19 - 5 = 14

Number of grey marbles to be transferred from Crate D to Crate C
= 13 - 1 p
= 13 - 1 x 5
= 13 - 5
= 8

Number of green marbles to be transferred from Crate D to Crate C
= 1 u - 7
= 14 - 7
= 7

Total number of grey and green marbles to be transferred from Crate D to Crate C
= 8 + 7
= 15

Answer(s): 15