Answer:
He'll be able to make 6 shelves.
Explanation:
In order to know haw many shelves he can make we need to take the total amount of material and divide it by the amount of material required for each shelve. We have:
total amount = 83 1/2 inches = 83.5 inches
amount per shelve = 12 3/4 inches = 12.75 inches
amount of shelves = total amount/amount per shelve
amount of shelves = 83.5/12.75 = 6.55 shelves
Since he can't make half shelves he'll be able to make 6 shelves and have left over material.