Tuesday, April 10, 2012

Mental arithmetic

This should be a simple problem - purchase four books at £7.99 each. These books are all in a 'Buy one get one half price offer'. An extra 20% off is available via a card. How much should it cost?

In quick terms it's £8 times 4, but with two at half price (equating to one full price book) it's really £8 times 3 or £24. Shift the decimal place for 10% to make £2.40. Times by 2 to make 20% £4.80. £24-£4.80 = £20-£0.80= £19.20 roughly speaking.

I thought £22.38 was a bit off. I paid the money walked away doing the sums in my head and coming to the answer above. Went back in and pointed this out. Taps on a calculator later and I was right. It turns out the till  had only halved the price of one book.

Can't trust tills.


Orphi said...

True fact: Every computer error is, in fact, a human error. ;-)

FlipC said...

Indeed. In this instance it seemed that three books from the shelves and one book from the bestseller list resulted in that one book not registering in the offer.

Given this was a store-wide offer one would think it easy to programme - Is it a book? Count the books and divide by two rounding down. Sort the books in price order and halve the number calculated.