Can anyone please help me with this? Any information is helpful. Thanks in advance!?

The length of time it takes to get through the security checks at a very large airport is a random variable with a mean u=20.6 minutes and standard deviation o(sigma)=8.4 minutes.
If a simple random sample of 36 passengers is to be observed going through security, what is the probability that the mean length of time for them to get through security will be under 18 minutes?

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