Problem with some easy programms :) C++

Hello all,my first post here />
I came across this forum and i think its a good place to post some problems i am dealing with as a newbie />.
Ok,so these are the programms i want to do:

1. Program that finds the smallest difference penultimate and last digit of 4 numbers you entered from the keyboard;

What is the easiest way to do it />? I know how to find last or penultimate digit of a number, but how to include the smallest difference into the program />, its confusing me lol There must be an easy way to do it :s

2. Write a program that finds the greatest common divisor two whole numbers a and b;

This sounds easy, i start it as
int a,b,n; (n is divisor i need to find)
printf("write first number");
scanf("%d",&a);
then same for the second one;
printf("enter n");
scanf("%d",&n);
if (a%n==0 & b%n==0) ... (and then idk:P ,this way we will find if a and b divides with n without leftover,but thats not what i need />)

Thanks in advance />,any help appreciated />

"noob"