Let uniform thickness of frame = x inch
so, length of picture (excluding frame) = (20 - 2x) inch
and, width of picture (excluding frame)= (8 - 2x) inch
Area of picture = (20 - 2x).(8 - 2x)
Also, given, Area of picture
so, (20 - 2x).(8 - 2x) = 115
x = 0.86, 13.14
since thickness of frame should be less than width of frame,
13.14 < 8
so, x cannot be 13.14 in.
therefore thickness of frame = 0.86 inch.