John has two jobs. For daytime work at a jewelry store he is paid $15,000 per month, plus a commission. His monthly commission is
normally distributed with mean $10,000 and standard deviation
$2000. At night he works occasionally as a waiter, for which his
monthly income is normally distributed with mean $1,000 and
standard deviation $300. John's income levels from these two
sources are independent of each other. For a given month, what is
the probability that John's commission from the jewelry store is
between $9,000 and $11,000?