Answer: He needs 20 minutes to install all of the fence.
Explanation:
Since we have given that
Dimensions of triangle he has to use for fencing are
15 feet, 8 feet, and 20 feet.
1) For making it a right triangle it must satisfy the "Pythagorus theorem" which states that

No, it will not be able to make a right triangle.
2) Joel cut the longest piece of wood in order to make a right triangle.
So, from above we get that

So, Longest side must be 17 feet.
It is takes him 30 seconds to install each foot of fencing,
Total perimeter of fencing will be

So, for 1 foot he needs = 30 seconds
For 40 feet , he will need

Hence, he needs 20 minutes to install all of the fence.