6. At the surface of the ocean, the water pressure on the submarine is the same as the air pressure above the water—about 15 lb/ in.2. Below the surface, the water pressure increases by about 9 lb/in.2 for every 20 ft of descent. Part A On another dive, the submarine descends at a steady rate from sea level. After 20 s the pressure gauge reads 100 lb/in.2. The rate of descent for the submarine, rounded to the nearest tenth is_________ft/s.