Question: A parachutist whose mass is 75kg drops from a helicopter hovering 4000m above the ground and falls toward the earth under the influence of gravity. Assume the gravitational force is constant. Assume also that the force due to the air resistance is proportional to the velocity of the parachutist with the proportionality constant b_1 = 15N sec/m when the chute is close and b_2 = 105N sec/m when the chute is open. If the chute does open 1 min after the parachutist leaves the helicopter after how many seconds will he hit the ground? Answer the differential equation way please!