One of the basic principles of doubleentry bookkeeping is that for any given transaction, the total of debits being posted should always equal the total of credits being posted. For instance, if we have made a cash sale with a net value of £100 and VAT is 20% then the double entry should be:
DR Cash  £120 
CR Sales  £100 
CR VAT Control  £20 
You can see that the debit side and the credit side have both posted a total of £120.
If every single doubleentry that we post balances in this way, then it shouldn’t matter whether there are ten transactions in a period, or ten million. The total of the debit entries should always equal the total of the credit entries. This means that at the end of the period when we balance off each ledger account and draw up a trial balance, the debit column should always give the same total as the credit column. That’s why it’s called a trial balance.
However, in the heat of an exam. It’s quite common to find that your trial balance doesn’t balance. This consequently means that you have made an error along the way.
Here are some useful tips on what to do if you find that the two columns in your trial balance don’t give you the same figure:

Doublecheck your addition
It is very easy under the pressure of an exam to make silly mistakes when adding up a long list of numbers. This is why it’s worth doublechecking (and even triplechecking) the accuracy of your addition. With a bit of luck, you might find that your two columns do actually balance. You just hadn’t added up one or the other correctly.
Once you are sure that you have indeed added up your two columns correctly, and they still don’t balance, we know there is an error somewhere and need to identify where it lies.

Recheck the logic of which items you have put on each side of the trial balance
A common source of error comes from placing an account balance on the wrong side of your trial balance, such as putting the sales figure on the debit side rather than the credit side.
Items to be particularly careful about are:
 Sales returns and purchase returns
 Discounts allowed and received
 VAT control account (being careful to identify if it is a balance payable or a balance receivable)
If you need to swap any balances onto the opposite side of the TB then you will need to recalculate the totals of the two sides.
If that doesn’t do the trick, then here are some other techniques for working out where your error might be:

Work out the difference between the two columns in your trial balance
You may have completely forgotten to put the figure from one of your ledger accounts onto the trial balance at all. If this is the case then your debit and credit columns will differ by the figure from that account.
By working out the difference between your debit and credit totals, and then searching for that difference in the original question, you might just spot which account balance you missed out. By inserting it you might get the magic balance you are looking for.

Divide the difference between the two columns by 2
Despite step 2, you might still have an account balance on the wrong side of the TB (it’s really easy to do!). You can hopefully imagine that if you have put a £1,000 balance on the debit side when it should be on the credit side, it will actually throw your columns out by a total of £2,000, double the figure you put on the wrong side.
If you divide your difference by 2 and then search for that figure, you may spot which account balance you have put on the wrong side of the TB. Use your DEAD CLIC principles to verify which is the correct side.

Divide the difference between the two columns by 9
Sometimes when we are processing lots of figures we can get digits muddled up. We might take a figure of £3,260 from the question and insert it into the Trial Balance as £3,620. This sort of mistake is so common it even has a name; it’s called a transposition error. Transposition errors are actually quite easy to spot as they always create a difference that divides exactly by 9. From above, £3,620 – £3,260 is a difference of £360 which divides exactly by 9.
If your difference divides exactly by 9, it’s likely you have made a transposition error somewhere. Carefully check each number in your TB to ensure that you have entered each of them correctly. Figures such as £73,839 are more likely to contain transposition errors than figures such as £20,000.
Hopefully those tips and techniques will give you a higher chance of successfully balancing your Trial Balance in your exam (and in your dreams!).