The stress in the material of a pipe subject to internal pressure varies directly with the internal pressure and the internal diameter of the pipe and inversely with the thickness of the pipe. The stress is 100  pounds per square inch when the diameter is 5 inches, the thickness is 0.75 inch, and the internal pressure is 25 pounds per square inch. Find the stress when the internal pressure is 40 pounds per square inch if the diameter is 8 inches and the thickness is 0.50 inch.