The proper divisors of 12 are 1, 2, 3, 4 and 6. A proper divisor of an integer N is a positive divisor of N that is less than N. What is the sum of the proper divisors of the sum of the proper divisors of 280?
1, 2, 4, 5, 7, 8, 10, 14, 20, 28, 35, 40, 56, 70, 140 add these together.....
