If the high temperature is 60°F and the low is 30°F, how many heating degree days occurred?

To calculate heating degree days (HDD), you can use the formula:

HDD = (Base Temperature) - (Average Daily Temperature).

A common base temperature used for heating degree day calculations is 65°F.

In this situation, first, you calculate the average daily temperature using the high and low temperatures:

Average Temperature = (High + Low) / 2

= (60°F + 30°F) / 2

= 90°F / 2

= 45°F.

Next, you subtract the average temperature from the base temperature:

HDD = 65°F - 45°F

= 20 HDDs.

This calculation shows that there are 20 heating degree days during the period when the high is 60°F and the low is 30°F. This is why the chosen answer of 20 HDDs is correct.