An equation expresses a relationship between (usually) two or more variables. For an equation to be a function there is only one output for each input.
In Economics, supply and demand are each represented by an equation.
Each equation has 2 variables: price () in dollars and quantity demanded ().
A value for can be substituted into the equation.
When the demand curve tells us the price
Let denote the bank balance after days.
This is the function where the value of the bank balance is dependent on , the number of days.
The value of the bank balance is after days.