stats problem

the time to complete a standardized exam is approximately normal with a mean of 70 minutes and a standard deviation of 10 minutes. how much time should be given to complete the exam so that 80% of the students will complete the exam in the time given?

My professor provided the answer, but not how to get it. Help?

Look up .80 in the body of the z table and note its corresponding z score.

Then, \(\displaystyle 10z+70=x\)

Solve for x by subbing in the z score you found.