A parachutist whose mass is 90 kg drops from a helicopter hovering 2500 m above the ground and falls toward the ground under the influence of gravity. Assume that the force due to air resistance is proportional to the velocity of the parachutist, with the proportionality constant b 1equals30 N-sec/m when the chute is closed and b 2equals90 N-sec/m when the chute is open. If the chute does not open until the velocity of the parachutist reaches 20 m/sec, after how many seconds will the parachutist reach the ground? Assume that the acceleration due to gravity is 9.81 m divided by sec squared.