Answer:
t = 31.6 s
Explanation:
In the exercise when he walks, as when going on the mobile sidewalk the speed is constant, so we can use the equations of motion
v = d/t
We have two situations
Walking
v1 = d / t1
On the mobile sidewalk
v2 = d / t2
He is raised on the sidewalk walked, this does not change the speed of either
vt = v1 + v2
We substitute and calculate
d / t = d /t1 + d /t2
1 / t = 1 / t1 + 1 / t2
1 / t = 1/50 + 1/86
1 / t = 0.02 + 0.0116
t = 31.6 s
You can see that as expected the time is less with greater speed