In most math series, the number of terms is denoted by "n" - if there are five items involved in the calculation, n=5.

The joke is that "if 'n' is the number of bikes you own, then the correct number of bikes to own is 'n+1', meaning regardless of how many bikes you have, you should get one more.

I have been told the same equation works with guitars and with *ahem* firearms. Or anything related to a hobby that people like to collect.