Let x be the common factor between the two lengths. The length of the shorter part is 3x and that of the remaining side is 5x. The lengths of these two parts should sum up to 30 in. The equation relating them is,
3x + 5x = 30
The value of x is 3.75. The shorter side is 3x or (3)(3.75) in. Thus, the shorter side is 11.25 inches.