October 31 days + November 30 days + December 31 days = 92 days.
October 31 days + November 30 days + December 1-4 = 65 days.
$100,000 x 92 = $9,200,000
$80,000 x 65 = $5,200,000
9,200,000 - 5,200,000 = 4,000,000
92-65 = 27
$4,000,000 / 27 = $148,148.15

You would need a balance of one hundred forty eight thousand, one hundred forty eight dollars and fifteen cents for the next 27 days in order to have an average daily balance of $100,000 for October through December.