Box C contains 4 white balls and 11 blue balls. Box D contains 73 white balls and 21 blue balls. How many blue balls and white balls must be moved from Box D to put into Box C so that 50% of the balls in Box A are white and 80% of the balls in Box D are white?

	Box C		Box D
	White balls	Blue balls	White balls	Blue balls
Before	4	11	73	21
Change	+ ?	+ ?	- ?	- ?
After	1 u	1 u	4 p	1 p

50% = ⁵⁰₁₀₀ = 12

80% = ⁸⁰₁₀₀ = 45

Number of white balls = 4 + 73 = 77 

Number of blue balls = 11 + 21 = 32 

1 u + 4 p = 77 --- (1)

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

(1) - (2)
(1 u + 4 p) - (1 u + 1 p) = 77 - 32
4 p - 1 p = 45
3 p = 45

1 p = 45 ÷ 3 = 15

From (2):
1 u + 1 p = 32
1 u + 1 x 15 = 32
1 u + 15 = 32

1 u = 32 - 15 = 17

Number of blue balls to be moved from Box D to Box C
= 21 - 1 p
= 21 - 1 x 15
= 21 - 15
= 6

Number of white balls to be moved from Box D to Box C
= 1 u - 4
= 17 - 4
= 13

Total number of blue and white balls to be moved from Box D to Box C
= 6 + 13
= 19

Answer(s): 19