Question

An elevator descends into a shaft at the rate of 3m/min. If the descent starts from 40m above the ground level, how long will it take to reach -20m?

Answer 1

Here, the total distance to be covered is 40m. above ground level and the 20m below. 
Hence, d = (40 + 20) m.
               = 60 m.
We know that: speed = 3 m. /min.
We also know that: $s= \frac{d}{t} \\ \\ t= \frac{d}{s} \\ \\ t= \frac{60}{3} \\ \\ t=20min$

Answer 2

speed=distance/time
we need time
time=distance/speed
Distance is 40 above ground+20 below gound=60m.(total)
Speed is 3m/min
substitute in formula
t=60/3
t=20min