A student builds a toy geyser for her engineering class that has height (in inches) of density

Question:

A student builds a toy geyser for her engineering class that has height (in inches) of density
fx(x) = 1/8 e-x/8
for x > 0,
and fx(x) = 0 otherwise. If she makes 20 such geysers, what is the rough probability that the average geyser height, among these 20 geysers, is 7 inches or more?