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Close but Wrong!

Just because an answer seems obvious doesn't mean it's right


Recently, a senior professional person (it was a lawyer) asked me a question about VAT.  "Sorry to sound dim, but if a product costs £120 including VAT and I want to work out the price without VAT, why can't I just take off 20%?" (to give £96).  I explained that this isn't how it works, and that you have to divide £120 by 1.2 instead, meaning the price before VAT was £100.  Her response: "Yes, but WHY?"

I could see her point.  When you add VAT to a price it seems like you just add 20%, yes?  So when subtracting VAT, why not just subtract 20%?  The answer: because sometimes our mathematical intuition when solving a problem is wrong.  The "adding" we're doing here isn't the same as when you add, say, £25 to the bill.

Some of the most counter-intuitive maths problems are ones where the wrong answer is very close to the correct one.  I think it’s often this closeness that makes the problems confusing.  If the VAT rate is set at 100% instead of 20%, then the flaw in the reasoning becomes more obvious.  Add 100% to £100 and you get £200.  Take off 100% and the price is zero.  (And as for Trump's tariffs of 145%....)

Here's a table of examples where the closeness of the wrong answer to the correct one increases the confusion:

Problem

Wrong
Answer

Correct
Answer

If the price including VAT is £120 what is
the price without VAT?

£96
take off 20%

£100
divide by 1.2

If your investment of £100 grows by 4% each year,
what’s it worth after five years?

£120
£100 x 1.2

£121.67
£100 x 1.04^5

If you drive to Brighton at 20mph and drive back at 30mph,
what's your average speed over the whole journey?

25mph
( 20+30)/2

24mph
2/ ( 1/20 + 1/30)

If a bat and ball cost £110, and the bat costs £100
more than the ball, what’s the price of the ball?

£10
£110 - £100

£5
£105 - £100

As with the VAT example earlier, sometimes the way to understand these mistakes is to use extreme examples to reveal the flaw in reasoning.  If your investment grows by 100% each year instead of 4%, it's easier to see the dramatic effect of compound interest (£100, £200, £400, £800...).  If you drive to Brighton at 20mph and return at 0mph your average speed isn’t 10mph, it’s zero – you would never get home!   And if the bat costs £0 more than the ball, you wouldn't say that the ball therefore costs £110.* 

 

BONUS PUZZLES

Here are two classic puzzles where incorrect reasoning gets really close to what seems to be the 'right' answer.

  • Three friends share a meal for £25.  They give the waiter £30, he gives them back £5, they leave a £2 tip.  The friends have now paid £9 each, the waiter has £2 as a tip.  £27+£2=£29. What happened to the missing pound?   Wrong answer: *Somebody stole it!*   Right answer:  They paid 3x£9=27, of which £25 went to the restaurant and £2 to the waiter.  There is no missing pound.
  • A sheikh leaves 17 camels to his three children, 1/2 to the eldest, 1/3 to the second and 1/9 to the third.  When he dies the children realise they can’t divide up the camels.  A passing traveller gives them his camel, so there are now 18.  The eldest takes 9, the next takes 6 and the third takes 2, which makes 17.  “So you didn’t need my camel after all,” says the traveller and he takes it back.   Wrong answer: *What magic is this?*  Right answer: the problem was that the sheikh's fractions didn't add up to 1, the flaw would have been much more obvious if the fractions had been, say, 1/3, 1/6 and 1/9.

 

* thanks to Colin Beveridge for suggesting this extreme example