Natural gas at 70F is flowing at a steady mass rate through a steel pipe with an inside diameter of 12 inches. Absolute pressure drops from 30 to 15 atm in 400 miles of horizontal pipe. M = 17, H = 0.01 cp. The volumetric flow rate per hour at 70F and normal barometric pressure is...

The supposed answer is 520000 ft3/hr. Kindly explain. Thank you.