500 guitar picks
order 15 boxes with 9 picks each
sell 19 boxes with 10 picks each
-> the store ordered about 150 picks (15 x 9 -> 15 x 10)
-> the store sold about 200 picks (19 x 10 -> 20 x 10)
it is reasonable, since they have 50 fewer picks which is about 450.
(second option)