A marble, a sticker and a bead costs $8.40 altogether.
The marble costs $2.30 less than the sticker.
The bead costs $2.30 more than the sticker.
How much do 4 beads cost?
Choose the cheapest item to be 1 u.
The marble costs $2.30 less than the sticker.
The cheapest item is the marble.

Cost of the marble = 1 u
Cost of the sticker = 1 u + 2.30
Cost of the bead = 1 u + 2.30 + 2.30 = 1 u + 4.60

Total cost of the marble, the sticker and the bead
= 1 u + 1 u + 2.30 + 1 u + 4.60
= 1 u + 1 u + 1 u + 2.30 + 4.60
= 3 u + 6.9

3 u + 6.90 = 8.40
3 u = 8.40 - 6.90
3 u = 1.50

1 u = 1.50 ÷ 3 = 0.50

Cost of 4 beads
= 4 x (1 u + 4.60)
= 4 x (0.50 + 4.60)
= 4 x 5.10
= $20.40

Answer(s): $20.40