A marble, a sticker and a bead costs $8.40 altogether.
The marble costs $1.80 less than the sticker.
The bead costs $1.50 more than the sticker.
How much do 2 beads cost?
Choose the cheapest item to be 1 u.
The marble costs $1.80 less than the sticker.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 1.80
Cost of the bead = 1 u + 1.80 + 1.50 = 1 u + 3.30

Total cost of the marble, the sticker and the bead
= 1 u + 1 u + 1.80 + 1 u + 3.30
= 1 u + 1 u + 1 u + 1.80 + 3.30
= 3 u + 5.1

3 u + 5.10 = 8.40
3 u = 8.40 - 5.10
3 u = 3.30

1 u = 3.30 ÷ 3 = 1.10

Cost of 2 beads
= 2 x (1 u + 3.30)
= 2 x (1.10 + 3.30)
= 2 x 4.40
= $8.80

Answer(s): $8.80