47.
It is 37.
37
Greatest . . . 37 Smallest . . . 23
19 x 41 = 779
It is: 37
42
The greatest prime factor of 40 is: 5
37 is a prime number between 30 and 40.
47 - 31 = 16
37
66
The greatest prime factor of 40 is 5. The greatest prime factor of 63 is 7. They do not have any prime factors in common.