Question

The cost, c, of a health club membership is related linearly to its length in months, m. If a 12-month membership costs $260 and an 18-month membership costs $380, which equation can be used to find the cost of a 30-month membership?
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Answer 1

Slope = (380 − 260)/(18 − 12) = 120/6 = 20, so c = 20m + b.
Use (12,260): 260 = 20·12 + b → b = 20.
Thus c = 20m + 20, and for m = 30: c = 20(30) + 20 = $620.