A sticker, a marble and a bead costs $8.10 altogether.
The sticker costs $2.10 less than the marble.
The bead costs $2.10 more than the marble.
How much do 4 beads cost?
Choose the cheapest item to be 1 u.
The sticker costs $2.10 less than the marble.
The cheapest item is the sticker.

Cost of the sticker = 1 u
Cost of the marble = 1 u + 2.10
Cost of the bead = 1 u + 2.10 + 2.10 = 1 u + 4.20

Total cost of the sticker, the marble and the bead
= 1 u + 1 u + 2.10 + 1 u + 4.20
= 1 u + 1 u + 1 u + 2.10 + 4.20
= 3 u + 6.3

3 u + 6.30 = 8.10
3 u = 8.10 - 6.30
3 u = 1.80

1 u = 1.80 ÷ 3 = 0.60

Cost of 4 beads
= 4 x (1 u + 4.20)
= 4 x (0.60 + 4.20)
= 4 x 4.80
= $19.20

Answer(s): $19.20