DAY 413
Credit: NPR Car Talk Puzzler
June 29, 2013

Tom and Ray, the Car Talk brothers, go out to eat with a group of friends. When the bill comes, the two brothers go to the bathroom and climb out the window. Realizing they've  been stiffed, the remaining group looks at the bill, which is $63. One of them correctly says, "If each of us chips in an extra two dollars, we'll have it covered."  How many were in the original group (i.e. including Tom and Ray)?

Answer:

Nine ............. The only two factors of 63 with a difference of two are 7 and 9. If Tom and Ray had paid their share, each of the nine people would have paid $7. Now, each of the remaining
seven people have to pay $9 each - a difference of $2.