Adam pays $8.30 for 2 marbles and 3 stickers.
A marble costs $3.40 more than 1 sticker.
What is the cost of 4 marbles and 2 stickers?
Choose the cheaper item to be 1 u.
1 marble costs $3.40 more than 1 sticker.
The cheaper item is the sticker.

Cost of the sticker = 1 u
Cost of the marble = 1 u + 3.40

Cost of 2 marbles = 2 x (1 u + 3.40) = 2 u + 6.80
Cost of 3 stickers = 3 u

Total cost of 2 marbles and 3 stickers
= 2 u + 6.80 + 3 u
= 2 u + 3 u + 6.80
= 5 u + 6.80

5 u + 6.80 = 8.30
5 u = 8.30 - 6.80
5 u = 1.50

1 u = 1.50 ÷ 5 = 0.30

Cost of 4 marbles and 2 stickers
= 4 x (1 u + 3.40) + 2 u
= 4 u + 13.60 + 2 u
= 4 u + 2 u + 13.60
= 6 u + 13.60
= 6 x 0.30 + 13.60
= 1.80 + 13.60
= $15.40

Answer(s): $15.40