Adam pays $12.80 for 2 marbles and 4 beads.
A marble costs $2.80 more than 1 bead.
What is the cost of 4 marbles and 2 beads?
Choose the cheaper item to be 1 u.
1 marble costs $2.80 more than 1 bead.
The cheaper item is the bead.

Cost of the bead = 1 u
Cost of the marble = 1 u + 2.80

Cost of 2 marbles = 2 x (1 u + 2.80) = 2 u + 5.60
Cost of 4 beads = 4 u

Total cost of 2 marbles and 4 beads
= 2 u + 5.60 + 4 u
= 2 u + 4 u + 5.60
= 6 u + 5.60

6 u + 5.60 = 12.80
6 u = 12.80 - 5.60
6 u = 7.20

1 u = 7.20 ÷ 6 = 1.20

Cost of 4 marbles and 2 beads
= 4 x (1 u + 2.80) + 2 u
= 4 u + 11.20 + 2 u
= 4 u + 2 u + 11.20
= 6 u + 11.20
= 6 x 1.20 + 11.20
= 7.20 + 11.20
= $18.40

Answer(s): $18.40