# Math Help - Finding linear cost function.

1. ## Finding linear cost function.

Im a little confused on how to come up with the linear cost function here:

Regrind, INC, regrinds typewriter platens. The variable cost to regrind each platen is $1.40. The total cost to regrind 70 platens is$400. Find the linear cost function to regrind platens. If regrind platens sell for \$9.40 each, how many must be reground and sold to break even ?

2. The linear cost has the model $C=aN+b$ where $C$ is the cost and $N$ is the number of typewriter platens reground.

$a = 1.4$ as it is a "per" cost giving $C=1.4N+b$ now substitute $(N,C)=(70, 400)$ into this equation to find $b$

3. Thanks...