A continuous random variable can take any value in a range: the hours you study in a week might be 3, 3.5, or 3.27. Because no single exact value has a probability, we describe it with a smooth curve, and probability is the area under the curve.
Pick a core class. You'll see a normal distribution for its weekly study hours, then use the 68-95-99.7 rule to read probabilities and percentiles off it.
Your choice isn't recorded anywhere.
Let X be the number of hours you study for in a week. Suppose X is normally distributed:
The shaded curve shows where study hours fall. Each band is one standard deviation wide, labelled with the share of students it holds:
68% of values lie within 1σ of the mean · 95% within 2σ · 99.7% within 3σ
Percentiles line up with standard deviations from the mean. Use the curve below.
Note: these numbers were made up and do not represent actual hours spent on each course, real or imagined!