Adam pays $19.80 for 3 beads and 4 marbles.
A bead costs $3.10 more than 1 marble.
What is the cost of 2 beads and 3 marbles?
Choose the cheaper item to be 1 u.
1 bead costs $3.10 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 3.10

Cost of 3 beads = 3 x (1 u + 3.10) = 3 u + 9.30
Cost of 4 marbles = 4 u

Total cost of 3 beads and 4 marbles
= 3 u + 9.30 + 4 u
= 3 u + 4 u + 9.30
= 7 u + 9.30

7 u + 9.30 = 19.80
7 u = 19.80 - 9.30
7 u = 10.50

1 u = 10.50 ÷ 7 = 1.50

Cost of 2 beads and 3 marbles
= 2 x (1 u + 3.10) + 3 u
= 2 u + 6.20 + 3 u
= 2 u + 3 u + 6.20
= 5 u + 6.20
= 5 x 1.50 + 6.20
= 7.50 + 6.20
= $13.70

Answer(s): $13.70