# Thread: help forming this equation

1. ## help forming this equation

A plumber charges £70 for 2 hours job and £145 for 5 hours job. Find $\frac{\mathrm{dc} }{\mathrm{d} t}$
How do i form an linear equation from this information.

I did this $C = \frac{70t}{2} + \frac{145t}{5}$ but it leads to $C = \frac{32}{5}t$

Therefore, i did not get the right answer.

2. ## Re: help forming this equation

You have two points

$p_1 = (2,70),~p_2 = (5,145)$

the slope is given by

$m = \dfrac{145-70}{5-2} = \dfrac{75}{3} = 25$

Using point slope form with $p_1$ we have

$y-70 = 25(x-2)$

$y = 25x + 20$