Is there a formula for calculating the average rate ?

The question is

The answer is 60/nQuote:

If a tree grows 5 feet in n years , What was the average rate in inches per year at which the tree grew during those years ??

I also got 60/n as the answer but i am not sure if my explanation for getting this answer is correct.

I got 60/n because the required value is average rate ininches per year. inches here is (12x5)/n = 60/n

Is this the correct reason for getting 60/n as the answer. Frankly i think this is a dumb question but it still appeared on GMAT.