I'm also a beginner to C, but from what I see you just need to put in a fflush. Here is a revised piece of code I made. I'm not entirely sure what your programs first line does or why a letter needs to be answered but here you go.
Code:
#include <stdio.h>
int main()
{
int n[50], num;
int i = 0, c = 0;
printf("Enter numbers (At end enter any letter): ");
while (scanf("%d", &n[i]))
{
i++;
}
printf("\n\tEnter a number: ");
fflush(stdin);
scanf("%d", &num); //This statement doesn't execute
printf("\n\tnum = %d", num);
fflush(stdin);
getchar();
}