1:40 means that the size of the object in the blue print is 40 times smaller than it will be when built. So 3.5 x 40 is 140 inches high. That can then be converted to feet 140 inches is 11.6667 feet. The .6667 can be converted to inches by multiplying it by 12, .6667 x 12 is 8.0. This means that the wall will be 11 feet and 8 inches high. Or just 140 inches.
