A stamp
collector decides to put all his stamps in a new album. If he puts 20
stamps on one sheet, there will be some left over; if he puts 23 stamps
on a sheet, there will be at least one empty sheet left in the album. If
the stamp collector is presented with an additional new album of the
same kind, each sheet holding 21 stamps, he will be left with a total of
500 stamps. How many sheets are there in the first album?

1) 11
2) 12
3) 13
4) 10
5) data insufficient