Gotta love z-scores questions.
The z-score is the normalized distance away from the mean for a given sample in a given population. These are very useful because they allow statisticians to compare things that are related but from different populations, such as test scores on different national standardized tests.
the formula for the z-score is
z = x-u/a
Since i cant write out greek letters, the u is a mu, standing for the mean of the population, while the a is a sigma standing for the standard deviation.
We can now calculate the z score for the above problem.
z = -1.208
Now what do we do with this number? In the back of your statistics book should be a score chart, with a large block of numbers. On one axis, usually the side, the z-scores will be listed in varying increments, find yours, then find the associated value on the table.
This value is the area under the curve of the normal distribution from negative infinity to the value. Since you want to find the chances that it takes longer than 18 minutes, you will want to find the area above this. To do this you have to subtract the value you have, from the total area under the curve, 1.
I did this and found that the second answer is correct, 0.1131.