I need help with this

Question

Cell Touch (a phone company) just breaks even when a customer uses his cell phone 40 minutes per month, but makes a profit of $20 when that customer uses 80 minutes per month. If P is the profit Cell Touch makes on a customer who uses his cell phone x minutes per month, complete a linear equation relating P and x below.

 

P=_x+_

Answer 1

So you need to think of each situation like a point on the graph.  If we let x be the number of minutes and y be the profit, the first situation would have the point (40, 0).  The second point then would be (80,20).  

Next we need to find slope using slop formula.   m = (y2 - y1)/(x2 - x1).  This would give us (20 - 0)/(80 - 40) = 20/40 = 1/2.  I personally like to use point-slope form to write an equation, but you could use slope intercept form next.

m = 1/2    Chose either point - I will use (40, 0).

y - y1 = m(x-x1)

y - 0 = 1/2(x - 40)

y = 1/2x - 20

P = 1/2x - 20

 

Hope that helps.