During a thunderstorm, 4 inches of rain fell in 2 1/2
hours. What was the rate of rainfall in inches per hour?

hourly rate.

Step-by-step explanation:

As others have noted, it is a simple mathematical division problem (4/2.5) and the average is 1.6 inches per hour.

However, if this is for sizing civil works such as culverts or plumbing storm water drainage systems to mitigate flooding, the highest rate per hour must be known and usually the highest ten minutes rainfall rate expressed as an hourly rate.

For example, if 4.0 inches fell over that 2.5 hours, nothing says that a rain band did not dump 3 inches in the first hour, (3 inches per hour cumulative) or 0.75 inches in a ten minute period (4.5 inches per hour rate), with the remainder of the storm tapering off to lesser rates. So the average is correct, but the peak hour is much more important for practical purposes.