Could you help giving me explanations on this problem ? Thank you.

Let X be a normal random variable with unknown mean and variance. We have following n = 8 samples for X: 7.12 7.23 6.78 6.78 5.99 6.58 7.90 7.05 l l l Compute the probability that the 40th percentile of X is contained in the interval (00, 6.78). (Give an exact answer.)