assuming that the 15 samples has a normal distribution. then the fraction of the sharpened pencil = 5 / 15 = 1 / 3
then solve the total number of pencil in the box
total pencil = (20) / ( 1 / 3)
total pencil = 20*3
total pencil = 60
so the number of unsharpened pencil = 60 - 20 = 40 pencils