Adam pays $6.10 for 2 marbles and 3 stickers.
A marble costs $1.80 more than 1 sticker.
What is the cost of 3 marbles and 4 stickers?
Choose the cheaper item to be 1 u.
1 marble costs $1.80 more than 1 sticker.
The cheaper item is the sticker.

Cost of the sticker = 1 u
Cost of the marble = 1 u + 1.80

Cost of 2 marbles = 2 x (1 u + 1.80) = 2 u + 3.60
Cost of 3 stickers = 3 u

Total cost of 2 marbles and 3 stickers
= 2 u + 3.60 + 3 u
= 2 u + 3 u + 3.60
= 5 u + 3.60

5 u + 3.60 = 6.10
5 u = 6.10 - 3.60
5 u = 2.50

1 u = 2.50 ÷ 5 = 0.50

Cost of 3 marbles and 4 stickers
= 3 x (1 u + 1.80) + 4 u
= 3 u + 5.40 + 4 u
= 3 u + 4 u + 5.40
= 7 u + 5.40
= 7 x 0.50 + 5.40
= 3.50 + 5.40
= $8.90

Answer(s): $8.90