An exponential function displays either a growth or decay behavior going from a low steep to a high steep.
well, clearly the LCD from the denominators of 5 and 10 is just 10, thus

Add the ratios 3:1 = 4
divide perimeter 128 by 4 = 32
multiply this by each ratio so 3:1 becomes 96length:32width
remember there are two lengths and two widths in the perimeter so divide by 2 to find the length = 48