Answer: He needs 36 cm to make the shelves.
Explanation:
Since we have given that
Length of first wood = 108 cm
Length of second wood = 72 cm
We need to find the greatest length so that he can make the shelves.
Greatest length = h.c.f. of 108 and 72 { 36 cm.
Hence, he needs 36 cm to make the shelves.