Formula

Is there a way to auto generate when a number is put it to change it to something else? I sell Jams and we sometime give discounts:

For example:

If 1 is typed in is there a way to change that to 7

If 2 is typed in is there a way to change that to 14

If 3 is typed in is there a way to change that to 20

So pretty much a jar cost $7 and every three jars $1 is taken off, or if there is a formula I can use that.