Find number abc

A group of girls walks into a bar. One of the women tells the bartender to line up a row of drinks for all of them. The gals lift their glasses and toast: "Here's to 51 days!" and they proceed to down their drinks.

Once again, they tell the bartender to "line 'em up" and once again they toast 51 days and down their drinks.

The bartender says: "I don’t get it. Why in the world are you toasting 51 days?"

One of the girl explains: "We just finished a jigsaw puzzle. It had written on the box '2-4 years' but we finished it in only 51 days!"