Here's how to tell if a given number is divisible by 11:

Add up the digits in the odd numbered positions to get a value. Add up the digits in the even numbered positions to get another value. Subtract the two values. If the result is 0 or a number that is divisible by 11, then the number itself is divisible by 11.

For example, 502557. The first value is 5 + 2 + 5 = 12. The second value is 0 + 5 + 7 = 12. Since 12 - 12 = 0, the number is divisible by 11.