The sum of the factors of 2^16 - 1 = 65535 is 299.
78
That depends on where you start and in what direction you proceed. If you list the prime factors in the order from smallest to largest, then the first two are 2 and 3, and their sum is 5.
42 has three prime factors: 2, 3 and 7
Mostly yes, but not always.
The two prime numbers whose sum is 75 are 37 and 38. 37 is a prime number because it has only two factors: 1 and 37. 38 is not a prime number because it has more than two factors (1, 2, 19, and 38).
Prime numbers have two factors. The sum of their proper divisors is always 1.
2 and 2. Prime factors needn't be distinct. Powers of the primes have only this prime as factor and 4 is a power of 2.
65 and 77
4nd 9.
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21 and 32
Prime numbers have only two factors which are themselves and one and they add up in the same way as composite numbers which have more than two factors