Thats 15 sq. feet (according to me)

let the length of the part of base, not included in side of Rectangle be X both side and the vertical side length be Y......

Now the rectangle has side lengths, Y and 10-2X.......

now use similar triangle property to find relation b/w Y and X

i.e. Y=6X/5

Hence area = XxY = 6X/5x(10-2X)

Use dA/dX=0 to get X=5/2

Therefore, A=15................