Since the expenses are fixed at $813.52/month, a 6-month emergency fund will be 6 times this value.
[tex]813.52\times6=4881.12[/tex]The fund must have $4881.12.
Since this will be funded in 18 months, we just need to divide the total amount of the fund by 18 to find how much must be saved per month.
[tex]\frac{4881.12}{18}=271.173333333\ldots\approx271.17[/tex]$271.17 must be saved monthly.