Crate D contains 4 grey balls and 15 white balls. Crate E contains 29 grey balls and 6 white balls. How many white balls and grey balls must be moved from Crate E to put into Crate D so that 50% of the balls in Crate A are grey and 80% of the balls in Crate E are grey?

	Crate D		Crate E
	Grey balls	White balls	Grey balls	White balls
Before	4	15	29	6
Change	+ ?	+ ?	- ?	- ?
After	1 u	1 u	4 p	1 p

50% = ⁵⁰₁₀₀ = 12

80% = ⁸⁰₁₀₀ = 45

Number of grey balls = 4 + 29 = 33 

Number of white balls = 15 + 6 = 21 

1 u + 4 p = 33 --- (1)

1 u + 1 p = 21 ---(2)

(1) - (2)
(1 u + 4 p) - (1 u + 1 p) = 33 - 21
4 p - 1 p = 12
3 p = 12

1 p = 12 ÷ 3 = 4

From (2):
1 u + 1 p = 21
1 u + 1 x 4 = 21
1 u + 4 = 21

1 u = 21 - 4 = 17

Number of white balls to be moved from Crate E to Crate D
= 6 - 1 p
= 6 - 1 x 4
= 6 - 4
= 2

Number of grey balls to be moved from Crate E to Crate D
= 1 u - 4
= 17 - 4
= 13

Total number of white and grey balls to be moved from Crate E to Crate D
= 2 + 13
= 15

Answer(s): 15