Suppose a 5-minute overseas call costs $6.95 and a 10-minute call costs $11.90. Write and solve a linear equation to find the cost for making and 18-minute call.

5 min = 6.95

10 min = 11.90

18 min = x

so, the difference between 5 min and 10 min = 11.90-6.95 = 4.95

15 min = 11.90+4.95 = 16.85

now, 4.95/5 = 0.99.

Then, 16 min = 16.85+0.99 = $17.84

17 min = 17.84+0.99 = $18.83

18 min = 18.83+0.99 = $19.82 Answer