suppose a 5 minute overseas call cost $5.91 and a 10- minute call costs $10.86. What is the cost, C, of M minutes

Question

how long can you talk if you have $12 to spend

Answer 1

Use direct variation with the possibility of a base charge.

 

5.91 = m(5) + b

10.86 = m(10) + b       subtract top equation from bottom

4.95 = 5m                   divide both sides by 5

0.99 = m

 

5.91 = 5(0.99) + b       simplify

5.91 = 4.95 + b            subtract 4.95 from both sides

0.96 = n

 

The base charge is 0.96

 

C = (0.99)M + 0.96

 

Substitute $12 for C and solve for M.