Need help with Subtraction and variables

Hello,

I have a few If-Then statements and the final one that I need is to basically "subtract 1 from every 5"

In my case, for every 5 miles traveled I need to subtract 1 year of life - its a wackly problem that I have.

How can I code this? Googling this results in a bunch of time subtraction links - that's not really what i need though.

Thanks!