Fisherman catching some yummy fish

What formula do you use? How do I solve this?

A survey of 250 lobster fishermen found that they catch an average of 32.0 pounds of lobster per day with a standard deviation of 4.0 pounds. If a random sample of 36 lobster fishermen is selected, what is the probability that their average catch is less than 31.5 pounds? Assume the distribution of the weights of lobster is normal.

Re: Fisherman catching some yummy fish

Try $\displaystyle P(\bar{X} <31.5) = P\left(Z< \frac{31.5-32}{\frac{4}{\sqrt{250}}} \right)$

Re: Fisherman catching some yummy fish

For samples of 36 the standard deviation of the means is 4/root36=4/6.