5%? You lucky fish!
It's quite easy.
The price you pay when buying, if it includes VAT, is 105%, right.
So you buy 30 bags of oranges, invoice price is R600 (including 5% VAT, and insert your own currency)
Easiest is to
first determine what is 1%:
So our equation is R600 = 105/100 (remember percent come from Latin
per cento, part of 100)
You want the RHS of the equation to be 1/100 (one percent);
To get the RHS of the equation to be 1/100, you have to divide both sides of the equation by 105;
so 600/105 = 5,714285714285714
and if we round it off, R5,71
So R5.71 = 1%
To get to 5%, we multiply 1% by 5
so R5.71 x 5 = R28,55
Getit so far? Good!
So when you bought the oranges, you paid R28.55 VAT, and the oranges actually cost R571.55
So VAT that you were charged on purchasing is R28,55
Then you sold those 30 bags of oranges for R1500, including VAT. Nice profit!
Same sum: determine what is 1%
R1500 = 105%
R1500 / 105 = 14,28571428571429 = 1%
we round it off to R14,29
To get 5%, we multiply 1% by 5
5X R14,29 = R71,45 = VAT on sales
Gettit? Good.
Now we can find the result of your formula:
(VAT On Purchasing )-(VAT On Sales)=VAT I have to pay for government
R28,55 - R71,45 =
- R42,90
Yikes!

Clearly, if you are a going concern (i.e. making a profit) having a negative value on VAT means the Government owes you, not other way around, so I suspect that your formula is wrong, it does not make sense.
Are you sure that the formula is not (VAT On Sales) - (VAT On Purchasing )= VAT I have to pay for government? In that case, then you would owe government R42,90.