could you help me please

This is a the question

The viscosity of a fluid can be measured in an experiment by dropping a small ball into a calibrated tube containing the fluid and observing the random variable X, the time it takes for the ball to drop the measured distance. Assume that X is normally distributed with a mean of 19 seconds and a standard deviation of 27.7 second for a particular type of liquid. What is the probability that the average time of 30 experiments will be less than 21.3 seconds