A marble, a sticker and a bead costs $8.10 altogether.
The marble costs $1.30 less than the sticker.
The bead costs $1.60 more than the sticker.
How much do 4 beads cost?
Choose the cheapest item to be 1 u.
The marble costs $1.30 less than the sticker.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 1.30
Cost of the bead = 1 u + 1.30 + 1.60 = 1 u + 2.90

Total cost of the marble, the sticker and the bead
= 1 u + 1 u + 1.30 + 1 u + 2.90
= 1 u + 1 u + 1 u + 1.30 + 2.90
= 3 u + 4.2

3 u + 4.20 = 8.10
3 u = 8.10 - 4.20
3 u = 3.90

1 u = 3.90 ÷ 3 = 1.30

Cost of 4 beads
= 4 x (1 u + 2.90)
= 4 x (1.30 + 2.90)
= 4 x 4.20
= $16.80

Answer(s): $16.80