Need help understanding an answer to a stats question

Can someone please explain an answer I received for the following stats quiz?

The background:

The cost of college has increased. Massively. But inflation has made all things cost more over time. In fact, annual college tuition increases are positively correlated with annual inflation (r=.5). To study the relationship between inflation and college tuition more deeply, researchers calculated the average percentage increase in tuition, over all US colleges, in each year for 1964 to 2008. From each value, they subtracted that year’s inflation percentage. This measures the “excess” increase in tuition over inflation. Over the 45 years of data, the average excess was 1.75 percentage points and the SD was 2.5. The histogram was normally distributed. The average annual inflation rate was 4.5 percent and the SD was 2.8. It was also observed that the excess in any given year was correlated with that year’s inflation rate (r=-.75). The scatter diagram is football shaped.

The Question:

Approximately how frequently did college tuition increase by an amount less than inflation?

The Answer:

My prof told me the answer is about 25 percent of the time. But he didn't really tell me the EXACT answer or how he arrived at it.

Please help!


Less is more. Stay pure. Stay poor.
How do you think you should approach this? What topics have you been covering in class? The key is the normal distribution?