A formula can be expressed as a worded problem or as an equation using words or letters.
Cost of hiring a van = number of days × £40 + £100
This formula allows you to calculate the cost of hiring a van based on the number of days it is needed for.
As you can see, there is a flat charge of £100 no matter how many days it is needed for.
For each day it is needed £40 is added to this cost.
Gareth is a electrician. He charges a call-out fee of £45 in addition to £15 for every hour on the job.
This can be re-written as an equation:
Wages = number of hours × £15 + £45.