Where in the problem does it say that there is no charge for 0 minutes? It's possible to have a charge for 0 minutes. One could express something like, "a service charges x dollars, plus y dollars per hour."

You're **not** going to get a **linear** equation that will satisfy this, because the slope between the 1st two points and the slope between the last two points aren't the same.

Taking the (0, 0) point out, find the slope between (.5, 15) and (1, 25), using the formula

.

Then plug in m, and plug in either of the points for x & y, into the slope-intercept form

to solve for b. (Incidentally, b tells how the service charge for 0 minutes.)

01