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.

Please help