Adam pays $15.40 for 3 beads and 4 marbles.
A bead costs $2.10 more than 1 marble.
What is the cost of 4 beads and 2 marbles?
Choose the cheaper item to be 1 u.
1 bead costs $2.10 more than 1 marble.
The cheaper item is the marble.

Cost of the marble = 1 u
Cost of the bead = 1 u + 2.10

Cost of 3 beads = 3 x (1 u + 2.10) = 3 u + 6.30
Cost of 4 marbles = 4 u

Total cost of 3 beads and 4 marbles
= 3 u + 6.30 + 4 u
= 3 u + 4 u + 6.30
= 7 u + 6.30

7 u + 6.30 = 15.40
7 u = 15.40 - 6.30
7 u = 9.10

1 u = 9.10 ÷ 7 = 1.30

Cost of 4 beads and 2 marbles
= 4 x (1 u + 2.10) + 2 u
= 4 u + 8.40 + 2 u
= 4 u + 2 u + 8.40
= 6 u + 8.40
= 6 x 1.30 + 8.40
= 7.80 + 8.40
= $16.20

Answer(s): $16.20