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 :)
