156
The sum of the factors is 156.
1,3,9,11,33,99
48
99
1 + 3 + 9 + 11 + 33 + 99 = 156
1, 3, 9, 11, 33, 99.
1, 3, 9, 11, 33, 99
49 of them.
Use the formula for the sum of an arithmetic sequence. Start with 11, end with 99; the interval is 2.
The sum of all the integers between 1 and 99 inclusive is 4950.
The sum of all odd numbers 1 through 99 is 9,801.