Answer:
7.0 m/s
Step-by-step explanation:
We can solve this using the 2 kinematic equations and you can use either:

So let's use the first one.
Vi = initial velocity
Vf = final velocity
a = acceleration due to gravity (constant 9.8m/s²)
t = time
The marble is dropped so this means that it is in free fall. The initial velocity is always 0m/s.
We are given then the following:
d = 2.5m
t = 0.71s
Vi = 0 m/s
a = 9.8m/s²
So we plug in the values we know and solve:

The answer:
r=2.745+8
r=10.745
If it takes .1 hours to walk 2/6 miles, then it takes one hour to walk 20/6 miles.
20/6 miles = 3.33 miles
So that is 3.33 miles per hour.