5/6
There are 12 inches in one foot or in other words 1 inches = 0.0833333333 feet so 1/20 inches would be 1/20 inches = 0.00416666667 feet. Follow the formula and you can convert any fraction value to feet.
According to the conversion units given above, if 1 foot equals 1/20 of an inch. There would be 20 feet in an inch of that diagram.