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Always the Same
Arrange the numbers 1 to 16 into a 4 by 4 array. Choose a number. Cross out the numbers on the same row and column. Repeat this process. Add up you four numbers. Why do they always add up to 34?
Mindreader
A little bit of algebra explains this 'magic'. Ask a friend to pick 3 consecutive numbers and to tell you a multiple of 3. Then ask them to add the four numbers and multiply by 67, and to tell you the last two digits of her answer. Now you can really amaze her by giving the whole answer and the three consecutive numbers used at the start.
Age 11 to 14 Challenge Level:
An excellent introduction to this sort of bellringing can be found in Ringing Elementary Minor Methods on Handbells by Frank H. King (the Cambridge University Bellringer) and in Wikipedia.
This is the first of three related problems. The second problem is You Owe Me Five Farthings, Say the Bells of St Martin's , and the third is When Will You Pay Me? Say the Bells of Old Bailey .
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NRICH is part of the family of activities in the Millennium Mathematics Project.
NRICH is part of the family of activities in the Millennium Mathematics Project.