In a rectangular figure, the opposite sides should be equal. If one of the sides measures 8 inches, the side opposite to this should also be 8 inches. We let x be the other dimension. The perimeter (P) is equal to,
P = 2 x 8 + 2x
Substituting the known values,
40 in = 16 + 2x
Solving for x gives an answer of 12 in. Thus, the other dimension is 12 inches.