The tensile strength of spot welds produced by a robot welder is normally distributed, with a mean

Question:

The tensile strength of spot welds produced by a robot welder is normally distributed, with a mean of 10,000 pounds per square inch and a standard deviation of 800 pounds per square inch. For a simple random sample of n = 4 welds, what is the probability that the sample mean will be at least 10,200 pounds per square inch? Less than 9900 pounds per square inch?