A 1 plus 1 is a particularly easy girl. Can also be referred to as 'the village bicycle'.

Let's face it, there really isn't anything easier than 1+1 now is there?
Bob: "So, you think I should go out with Suzy?"
Steve: "Oh yeah, you'll totally get some that girl is a real 1 plus 1."
by Lochnivar November 14, 2009