You can only get one 1/7 inch piece out of a 5/21 inch piece.
To find this, first give them common denominators. 1/7 is equal to 3/21 (multiply both parts by a factor of 3). Now we can subtract the amount a number of times until we don't have enough left.
5/21 - 3/21 = 2/21
After cutting only one piece, we do not have enough for another of that size. Therefore, the answer is 1.
