Luke has 25 units of fencing which is the largest rectangle for which he can use the fencing?

The perimeter of Rectangle should be equal to 25 units as the length of fence is 25 units.

So here is one example,

length of rectangle is 10 units and width is 2.5 units and perimeter is 2 * (l+b)

=> 2 * (10+2.5)

=> 2*12.5

=> 25 units.