x = sleep time, which is normally distributed with mean 6.9 and standard deviation 0.68.

We are trying to find P(x > 7.5). To use standard normal probability table (z-scores), we must convert that inequality to a z-score: (x - mean)/(standard deviation/square root (sample size))/

So the inequality becomes P(z > (7.5 - 6.9)/(0.68/square root(15)).

We can then look up that probability from the standard normal probability table. Since probabilities in that table are cumulative less than, you will use the fact that P(z > value) = 1 - P(z < value).