Three people enjoy a meal at a Thai restaurant. The waiter brings the bill for £30 so each person pays £10.
Later the chef realises that the bill should have only been £25 so he sends the waiter back to the table with £5. The waiter was not very good at Maths and could not figure out how to divide the £5 so he gave each person a £1 and kept £2 for himself.
So....the three people have paid £9 each for the meal.
3 x £9 = £27
The waiter kept £2
£27 + £2 = £29
What happened to the other pound? Does this make sense?
This activity is suitable for students of mathematics all around the world. Use the button below to change the currency symbol used to make it more relevant to your students. You may wish to choose an unfamiliar currency to extend your students' experience.
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Danny Baker on his wonderful BBC radio 5 live programme suggests a unique solution to the missing pound puzzle.
The final paragraph of the story should read:
The waiter kept £2
£27 − £2 = £25, the correct cost of the meal.
Here is a similar puzzle from Thailand: "You borrow money from your Dad (500 baht) and your Mom (500 baht) to buy a phone that costs 970 baht. You then you have 30 baht change from the shop so you return 10 baht to Dad and 10 baht to Mom and you keep 10 baht yourself. But 490 + 490 = 980 and the 10 baht that you keep totals 990 baht. Where is the missing 10 baht?"
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Have you read Craig's book yet?
Craig Barton must surely be the voice of Mathematics teachers in the UK. His wonderful podcasts interviewing the industry experts have culminated in this wonderful book. As Craig says: "I genuinely believe I have never taught mathematics better, and my students have never learned more. I just wish I had known all of this twelve years ago..." more...
"How I wish I'd taught Maths" is an extraordinary and important book. Part guide to research, part memoir, part survival handbook, it’s a wonderfully accessible guide to the latest research on teaching mathematics, presented in a disarmingly honest and readable way. I know of no other book that presents as much usable research evidence on the dos and don’ts of mathematics teaching in such a clear and practical way. No matter how long you have been doing it, if you teach mathematics—from primary school to university—this book is for you." Dylan Wiliam, Emeritus Professor of Educational Assessment, UCL.
Casio Classwiz Calculator
There is currently a lot of talk about this new calculator being the best in its price range for use in the Maths classroom. The new ClassWiz features a high-resolution display making it easier to view numerical formulas and symbols but it isn't a graphical calculator as such (it has the capacity to draw graphs on your smart phone or tablet, via a scannable QR code and an app).
As well as basic spreadsheet mode and an equation solving feature you also get the ability to solve quadratic, cubic or quartic polynomial inequalities and the answer is given just as it should be written down, using the correct inequality symbols!
This calculator has a high-performance processor and twice the memory of previous models ensuring speedy operation and superior computational power.more...
Teacher, do your students have
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A man had an apple stall and he sold his larger apples at 3 for a pound and his smaller apples at 5 for a pound.
When he had just 30 apples of each size left to sell, he asked his son to look after the stall while he had lunch. When he came back from lunch the apples were all gone and the son gave his father £15.
The father questioned his son. "You should have received £10 for the large apples and £6 for the 30 small apples, making £16 in all."
The son looked surprised. "I sold them all at the average price of 2 small and 2 large for £1. Four into 60 goes 15 times so I am sure £15 is correct.
Where is the missing pound?
This extension is adapted from a puzzle in Amazing Brain Teasers by Erwin Brecher
The average cost of the large apples is £1 ÷ 3 = 33⅓p.
The average cost of the small apples is £1 ÷ 5 = 20p.
So the 2 small and 2 large apples should have been sold for
33⅓p + 33⅓p + 20p + 20p = £1.06⅔
to earn the £16