1/4 is the shortest piece of wood.
The common denominator for all the fractions is 60.
Multiply the numerator by how many times the denominator goes into 60.
The fractions turn into 1/3 = 20/60, 2/5 = 24/60, 3/10 = 18/60, 1/4 = 15/60
15 is the smallest number, so 15/60 is the smallest piece.
