Adam pays $17.80 for 2 marbles and 4 beads.
A marble costs $3.80 more than 1 bead.
What is the cost of 3 marbles and 2 beads?
Choose the cheaper item to be 1 u.
1 marble costs $3.80 more than 1 bead.
The cheaper item is the bead.

Cost of the bead = 1 u
Cost of the marble = 1 u + 3.80

Cost of 2 marbles = 2 x (1 u + 3.80) = 2 u + 7.60
Cost of 4 beads = 4 u

Total cost of 2 marbles and 4 beads
= 2 u + 7.60 + 4 u
= 2 u + 4 u + 7.60
= 6 u + 7.60

6 u + 7.60 = 17.80
6 u = 17.80 - 7.60
6 u = 10.20

1 u = 10.20 ÷ 6 = 1.70

Cost of 3 marbles and 2 beads
= 3 x (1 u + 3.80) + 2 u
= 3 u + 11.40 + 2 u
= 3 u + 2 u + 11.40
= 5 u + 11.40
= 5 x 1.70 + 11.40
= 8.50 + 11.40
= $19.90

Answer(s): $19.90