Adam pays $12.20 for 3 beads and 2 marbles.
A bead costs $3.90 more than 1 marble.
What is the cost of 4 beads and 3 marbles?
Choose the cheaper item to be 1 u.
1 bead costs $3.90 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 3.90

Cost of 3 beads = 3 x (1 u + 3.90) = 3 u + 11.70
Cost of 2 marbles = 2 u

Total cost of 3 beads and 2 marbles
= 3 u + 11.70 + 2 u
= 3 u + 2 u + 11.70
= 5 u + 11.70

5 u + 11.70 = 12.20
5 u = 12.20 - 11.70
5 u = 0.50

1 u = 0.50 ÷ 5 = 0.10

Cost of 4 beads and 3 marbles
= 4 x (1 u + 3.90) + 3 u
= 4 u + 15.60 + 3 u
= 4 u + 3 u + 15.60
= 7 u + 15.60
= 7 x 0.10 + 15.60
= 0.70 + 15.60
= $16.30

Answer(s): $16.30