JEE Advanced 2020 Full Test 5 Paper 1

One end of a string of length L=15m is tied to the ceiling of an elevator accelerating upwards with an acceleration 20m∕s2. The other end of the string is free. The linear mass density of the string varies linearly from 0 to λ0 from bottom to top. Find the time taken by a pulse (in seconds) to reach from bottom to the top. (g=10ms−2)