X=9.
To determine the last number of a Social Insurance Number, first put an alternating sequence of the numbers 1 and 2 under the first eight digits:
417 368 90
121 212 12
Next, multiply each digit from the SIN by the 1 or 2 below it:
417 368 90
121 212 12
427 66 (16)* 90
*If the product of the two numbers is a two digit number, add the two digits together:
417 368 90
121 212 12
427 66 (7) 90
Add the products of the multiplications:
4+2+7+6+6+7+9+0= 41
Subtract the sum of the products from the multiple of 10 greater than or equal to it:
50-41=9
So, if someone wants to create a dummy SIN, (and they don't know the formula), they have a 10% chance of creating a valid one.
