This one is similar to the one mentioned by csm_714
There was a 16 year old girl .. enter 16 on calculator
And she had sex with 19 different guys.. multiply this number with the 16 mentioned beforhand.
What does this make her?
a 304. (a hoe)
This loses its humor when someone mentions that the word perceives different meaning due to its spelling and that there isn't really anything funny about being a garden tool.