



 
Hi Tom, I can show you one place where there is an error in your reasoning but I'm not sure that fixing it completely solves the problem. When you say "we have taken £3179.32 including VAT, which would be £2543.45 excluding VAT (3179.3220%=2543.45." you are subtracting 20% of £3179.32 which is the orders including the VAT but the 20% should should be on the before VAT orders. You know that \[\mbox{(the before VAT orders + 20%) }= £3179.32\] or \[\mbox{(the before VAT orders)} \times 1.20 = £3179.32\] which means that \[\mbox{(the before VAT orders}) = \frac{ £3179.32}{1.20} = £2649.43.\] But £2649.43  £2578.25 = £71.18. Is it possible that the missing order has a value of £71.18? Penny  


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