21 into 15 best min fragment found: 5
  Cut up 21 sticks of length 15 like this:
    6 x (7 + 8)
    6 x (6 + 9)
    9 x (5 + 5 + 5)
  Reassemble as 15 sticks of length 21 like this:
    3 x (5 + 8 + 8)
    6 x (5 + 7 + 9)
    6 x (5 + 5 + 5 + 6)