Question

Matthew has twice as many stickers as David. How many stickers must Matthew give David so that they each will have 120 stickers?

Answer 1

M is Matt's sticker count, D is David's:

The equations:

The average sticker count has to be 120, so:

(M + D) / 2 = 120

Matt has twice as many as David:

M = 2D

Substitute for M in the average, so:

(2D + D) / 2 = 120

Solving you get:

3D = 240

D = 80 and 120 - 80 is 40, so Matt has to give him 40 stickers.

Hope this helps :)