Math question

Consider a large clothing shop in Sydney. Suppose it is known that the number of business suits sold per day is normally distributed with a mean, μ = 23 and standard deviation, σ = 10.  Mr Wood is employed to sell business suits. The number of business suits he sells each day for 40 days is recorded and the mean number per day calculated.

What is the probability that Mr Wood’s average daily sales will be less than 19 business suits