Posted by Betty on Sunday, August 12, 2012 at 8:35pm.
It is not mentioned that the distribution is normal. We will assume that it is, either by assumption, or by an approximation using the central limit theorem (which preferably applies to samples of 30+).
With the assumption, we can see that the variance is σ²/n=0.4²/16=0.01, hence σ=0.1.
Look up the Normal distribution table for 1-tail probability of 1σ which gives the probability of 1-0.8413=0.1587 for less than μ-1σ=3.1-0.1=3.0 minutes.
Can you take it from here?
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