10=1+9      1x9=9
  =2+8      2x8=16
  =3+7      3x7=21
  =4+6      4x6=24
  =5+5      5x5=25
↑ Left: the possible measurements of the two fences, Right: the possible areas
out of the areas calculated, 25 sq feet is the maximum. :)
Ans: 25 square feet