68 = 2 * 2 * 17
17 + 2 = 19
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.
The sum of the two largest prime numbers less than 71 is 128.
5
The largest prime less than 20 is 19. The largest prime over 60 is 61. The sum of these two numbers is 80.
78
The sum of the factors of 2^16 - 1 = 65535 is 299.
269
The largest is 313. The other two prime factors are 2 and 11.
They are: 89+97 = 186
The largest factor that 12 and 30 have in common is the product of the two prime factors they have in common.
17, 23.
42 has three prime factors: 2, 3 and 7