How to tell a number whether it is divided by 11.

Here's how to tell when a number up to 1,000 divides exactly by 11.
If it's a 2 digit number, the digits will be the same. (E.g.33 or 77)
If it's a 3 digit number: add the two outside digits and take away the middle digit. If the answer is 0 or 11 then the number will divide by 11. Examples: 451: 4+1-5 =0 so 451 will divide by 11. 869: 8+9-6=11 so 869 will divide by 11.