What is an equation?
An equation is just a mathematical rule which is used to calculate something.
An equation can be written in a very plain English way, for example the cost of renting a motorbike could be a fixed fee of $50 plus $25 per day.
So, the cost of renting the motorbike could be described as:
Cost = $50 + $25 per day
Equations generally use letters to allow for unknown values. So, in our example we could use the letter C to denote the total cost of the rental in dollars and the letter d to denote the number of days the motorbike will be rented for.
In which case, our motorbike rental equation becomes:
C = 50 + 25d
All we're saying here is that we multiply the number of days (d) by 25 & add 50 and that gives us the cost, in dollars, of renting the motorbike.