Answer: He will use 5 feet long to have the least amount of unused glass.
Step-by-step explanation:
Since we have given that
There are four pieces of glass i.e.
6 feet long, 5 feet long, 3 feet long, 7 feet long .
If the length of the glass he needs to cut is feet long,
Then,
So, Mark will use 5 feet long he should use cut to have the least amount of unused glass.