High-Low Points Method. Sam's Supermarket considers electricity a mixed cost. By using a scatter chart, Sam has determined that there is a relationship between electricity expense and the number of hours per month the store is open. During the past year, electricity expense totaled $3,600 in the month that the store was open 600 hours and $2,100 in the month that it was open 300 hours. What fixed monthly cost and hourly rate should Samuse to estimate electricity expense for the upcoming year?