When you have purchased a combination lock but have lost the numbers on the back that came with the device, you will need to purchase another lock or crack the lock yourself. While it is illegal in the United States to gain entry to someone's personal belongings by bypassing a lock, you can complete the task if the lock is yours. The process requires a bit of simple math and patience, but the reward can be priceless.

Turn the combination lock to "0." Pull the latch up to create tension.

Turn the lock slowly to the right while still pulling up on the latch. As you turn, there will be a stopping point when you can't turn it any further.

Write the first number where the lock stops. You will need this number to form a list of 12 numbers, which will deduce the last number on the lock's combination. If the dial stops between two numbers, count that as a half stop and write down a "0.5." For example, if it stops between 8 and 9, write "8.5."

Release the tension on the latch slightly. Turn the dial softly to the right with force on the latch. Write down the number where the dial stops turning. Continue the process until you finish a list of 12 numbers.

Note that five whole numbers and seven half numbers should be listed. Circle all whole numbers. Four or five of the whole numbers will have the same digit. For example, if you have a series of 4, 14, 24, 34 and 39, four of those numbers have a 4 as the last digit in the number. Asterisk the number that doesn't have a 4 in it. In this case that number would be 39. This is the last number in the combination.

Take the result from Step 5 (39) and divide it by four. Eliminate the remainder. For example, 39 divided by four is 9.75. Eliminate the 0.75. The remainder, though, is 3. You find this by multiplying 0.75, or whatever your remainder is, by four.

Start with your remainder. For the purpose of this example, let's use 3. Add increments of four to your remainder, until you can't go any higher. Your list will read, "3, 7, 11, 15, 19," and so on. The list shouldn't include numbers higher than 39, as that is the highest number on combination locks. You will be able to find the combination's first number from this list.

Subtract or add 2 to the list of numbers from Step 7. If the remainder was 0 or 1, add 2 to each number from the Step 7 list. If the remainder was 2 or 3, subtract 2 from the numbers in Step 7. The list from Step 7 will now read "1, 5, 8, 13, 17," and so on.

Use the list from Step 7 to figure out what the first number of the combination is. Step 8 will be the second number. Run through all possibilities, ending with what you have already determined to be the final number of the three-number sequence. In this case, 39. Using the examples already covered, you'd start with 1-3-39, 1-7-39, 1-11-39, and so on, until all the possibilities have been attempted. It will take several minutes before you have attempted all the possible combinations, though you may discover the combination on the first try.