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| Jan 15, 2022 | » | 020. Factorial Digit Sum
4 min; updated Jan 15, 2022
Problem Statement \(n!\) means \(n \times (n - 1) \times … \times 3 \times 2 \times 1\). For example, \(10! = 10 \times 9 \times … \times 3 \times 2 \times 1 = 3628800\), and the sum of the digits in the number \(10!\) is \(3 + 6 + 2 + 8 + 8 + 0 + 0 = 27\). Find the sum of the digits in the number \(100!... |
| Feb 6, 2021 | » | 021. Amicable Numbers
8 min; updated Feb 6, 2021
Problem Statement Let \(d(n)\) be defined as the sum of proper divisors of \(n\) (numbers less than \(n\) which divide evenly into \(n\)). If \(d(a) = b\) and \(d(b) = a\), where \(a \neq b\), then \(a\) and \(b\) are an amicable pair and each of \(a\) and \(b\) are called amicable numbers. For example, the proper divisors of \(220\) are \(1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110\); therefore \(d(220) = 284\).... |