A car hire company makes the following charges:

•A one-off charge of £20:
•A charge of £25 for each day that the car is hired.
•A charge of 10p for each mile that car is driven.

Write down a formula that gives the total cost (£C) of hiring a car for D days and driving M miles. Use your formula to find the cost of hiring a car for 3 day and driving 200 miles.

how do i work this out ?

I would say
Cost = 20 + 25D + .1M

so if D=3 and M=200
Cost =20+25(3)+.1(200)
=....