Note that 
, i.e. a student that completes the exam in one hour falls exactly one standard deviation (in the negative direction) from the mean.
Now, you're looking for the proportion of students that fall below this, i.e.
. The empirical rule states that approximately 68% of a normal distribution falls within one standard deviation of the mean. That is,
This means that the proportion of students that fall outside this range is
Because the distribution is symmetric, it follows that
This means the percentage of students that complete the exam in under an hour is 16%.
Now, you're looking for the proportion of students that fall below this, i.e.
This means that the proportion of students that fall outside this range is
Because the distribution is symmetric, it follows that
This means the percentage of students that complete the exam in under an hour is 16%.