There were some brown erasers and grey erasers. The erasers were packed into 2 bags. At first, Box D contained 300 erasers and 30% of them were grey erasers. Box E contained 720 erasers and 60% of them were grey erasers. How many brown erasers and grey erasers in total must be moved from Box D to Box E such that 20% of the erasers in Box D are brown and 50% of the erasers in Box E are grey?

	Box D		Box E
Total	300		720
	Grey erasers	Brown erasers	Grey erasers	Brown erasers
Before	90	210	432	288
Change	- ?	- ?	+ ?	+ ?
After	4 u	1 u	1 p	1 p

Number of grey erasers in Box D at first
= 30% x 300
= ³⁰₁₀₀ x 300
= 90

Number of brown erasers in Box D at first
= 300 - 90
= 210

Number of grey erasers in Box E at first
= 60% x 720
= ⁶⁰₁₀₀ x 720
= 432

Number of brown erasers in Box E at first
= 720 - 432
= 288

Box D in the end
20% = ²⁰₁₀₀ =¹₅
Grey erasers : Brown erasers = 4 : 1

Box E in the end
50% = ⁵⁰₁₀₀ =¹₂
Grey erasers : Brown erasers = 1 : 1

Total number of grey erasers = 4 u + 1 p

4 u + 1 p = 90 + 432
4 u + 1 p = 522 

1 p = 522 - 4 u --- (1)

Total number of brown erasers = 1 u + 1 p
1 u + 1 p = 210 + 288
1 u + 1 p = 498

1 p = 498 - 1 u --- (2)

(2) = (1)
498 - 1 u = 522 - 4 u
4 u - 1 u = 522 - 498
3 u = 24

1 u = 24 ÷ 3 = 8

Total number of brown erasers and grey erasers that must be moved from Box D to Box E
= 300 - 5 u
= 300 - 5 x 8
= 300 - 40
= 260

Answer(s): 260