Step 1: Knowing thy lock.
First of all, there is a dial, a body, and a clasp (The U shaped thing on the top that comes out when unlocked).
When you open your lock you would normally spin the dial right a few times to reset, then go to your first number. Then, you would turn left, passing your first number once, then stopping on your second number. Finally, you would turn right again to your third, and final number and pull on clasp.
Okay, now you know thy lock.
Step 2: Finding your third number
So what you do, is take your pencil and cross out all the numbers with decimals (.5, .25, .75)
now you should have 5 numbers. If not, you didn't use enough brain cells to get the right middle number. Try again. But if you do have 5, you will notice 4 of the 5 end in the same number. Cross these out to. That should leave one number. In the case of the list above, that number would be 19.
That final-left number, is your third number.