the calculation I came up with is

c=5l +500

How did you arrive at this formula?

You plugged in various numbers, such as those suggested earlier, and did the computations, paying close attention to your steps and reasoning:

. . . . .L = 50, so extra length is 50 - 50 = 0:

. . . . .C = 500 + 5(0) = 500 + 5(50 - 50)

. . . . .L = 51, so extra length is 51 - 50 = 1:

. . . . .C = 500 + 5(1) = 500 + 5(51 - 50)

. . . . .L = 52, so extra length is 52 - 50 = 2:

. . . . .C = 500 + 5(2) = 500 + 5(52 - 50)

. . . . .L = 55, so extra length is 55 - 50 = 5:

. . . . .C = 500 + 5(5) = 500 + 5(55 - 50)

. . . . .L = 60, so extra length is 60 - 50 = 10:

. . . . .C = 500 + 5(10) = 500 + 5(60 - 50)

...and so forth. What pattern did you see?

Eliz.