Math
posted by Julia .
A homeowner empties her hot water heater every 36 days and changes the filter in the heat pump every 48 days. She did both on Friday September 17, 2012. When again will she do both on the same day? (give the day of the week, the date, month, and year)

This is like finding the LCM of 36 and 48
36 = 2x2x3x3
48 = 2x2x2x2x3
LCM = 2x2x2x2x3x3 = 144
So it will happen 144 from Sept 17, 2012
BTW, Sept 17, 2012 was a Monday , not a Friday , so I will let you sort out the rest.
http://www.searchforancestors.com/utility/dayofweek.html