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

solve,

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.