Answer:
The smallest sample size that we should consider is 68.
Step-by-step explanation:
We have that to find our
level, that is the subtraction of 1 by the confidence interval divided by 2. So:

Now, we have to find z in the Ztable as such z has a pvalue of
.
That is z with a pvalue of
, so Z = 1.645.
Now, find the margin of error M as such

In which
is the standard deviation of the population and n is the size of the sample.
Standard deviation of 10 minutes.
This means that 
What is the smallest sample size that we should consider?
This is n for which M = 2. So






Rounding up:
The smallest sample size that we should consider is 68.