A parachutist whose mass is 75 kg drops from a helicoptor hovering 2000m above the ground. Falls under the influence of gravity. Assume force due to air resistance is prop to velocity of the parachutist, with constant b1= 30 N-sec/m when the chute is closed and b2= 90 N-sec/m when the chute is open. If the chute does not open until the velocity of the parachutist reaches 20 m/s after how many sec will she reach the ground.

when I tried this the first time I was off by 11 secs. Cannot find my mistake.

Question

A parachutist whose mass is 75 kg drops from a helicoptor hovering 2000m above the ground. Falls under the influence of gravity. Assume force due to air resistance is prop to velocity of the parachutist, with constant b1= 30 N-sec/m when the chute is closed and b2= 90 N-sec/m when the chute is open. If the chute does not open until the velocity of the parachutist reaches 20 m/s after how many sec will she reach the ground.

when I tried this the first time I was off by 11 secs. Cannot find my mistake.