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 :)