If $2.4$ inches of rain fell between January and April and the annual rainfall is about $12$ inches,
the remainder must have fallen from May through December.

Since $12 − 2.4 = 9.6$, the rainfall from May through December must be $9.6$ inches.
If $9.6$ inches is taken as a fraction of the total average annual rainfall of $12$ inches,
what percentage of the annual rainfall after April does this fraction represent?
$\frac{9.6}{12} \rightarrow \frac{96}{120} \rightarrow \frac{4}{5}$
Covert the fraction answer to a percentage to find the answer.