The average number of stickers that Adam and Bryan have is 4.
The average number of stickers that Bryan and Charlie have is 5.
The average number of stickers that Charlie and Adam have is 7.

1. How many stickers does Adam have?
2. How many stickers does Bryan have?
3. How many stickers does Charlie have?

(a)
Let the number of stickers that Adam has be A.
Let the number of stickers that Bryan has be B.
Let the number of stickers that Charlie has be C.

A + B = 2 x 4 = 8 --- (1)
B + C = 2 x 5 = 10 --- (2)
A + C = 2 x 7 = 14 --- (3)

(A + B) + (B + C) + (A + C) = 8 + 10 + 14
2 x (A + B + C) = 32
A + B + C = 32 ÷ 2
A + B + C = 16 --- (4)

Substitute (2) into (4).
A + B + C = 16
A + 10 = 16

A = 16 - 10 = 6

Number of stickers that Adam has = 6

(b)
Substitute A = 6 into (1).
A + B = 8
6 + B = 8 

B = 8 - 6 = 2

Number of stickers that Bryan has = 2

(c)
Substitute Bryan's stickers = 2 into (2).
B + C = 10
2 + C = 10

C = 10 - 2 = 8

Number of stickers that Charlie has = 8

Answer(s): (a) 6; (b) 2; (c) 8