Moderate# Interpreting the Definite Integral

APCALC-J9NSSB

There are 120 letters in a mailbox at 12:00 pm. Over the next 5 hours, the number of letters in the mailbox changes at a rate modeled by the function $r(t)$, where $r(t)$ is integrable and measured in letters per hour, and $t$ is the time in hours since 12:00 pm. Of the following statements, which one is the best interpretation of $\int _{ 0 }^{ 3 }{ { r(t)dt } } $?