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Doesn't Add Up

Age 14 to 16 Challenge Level:

This problem is available as a printable worksheet: Doesn't Add Up.

Why do this problem

This problem forces attention on what happens when we calculate area by dissection into known shapes and the care we should take in our justification. In problem solving diagrams can often be misleading, and not just diagrams we draw for oursleves. There is a need to question and check that the representation is meaningful and conveys the message accurately. This problem brings that idea to the fore.


Possible approach 

You may be interested in our collection Dotty Grids - an Opportunity for Exploration, which offers a variety of starting points that can lead to geometric insights.

Many students will need to dissect the 8 by 8 square into the four pieces to begin to get a feel for the possible source of the discrepancy. Although students want to know the answer (that's what motivates the problem-solving effort) it is the process of finding a route to an answer which provides the real long-term benefit.

Once someone says out loud where to look for the discrepancy the problem is over.

To avoid this, encourage students who are ready to develop a convincing argument to share and then to move on to the challenge of creating similar problem. When ready move to a whole group discussion emphsising the need for convincing arguments. The extension activity also gives scope to those who see what is happening quickly.

A possible context for this problem is the so-called "infinite chocolate trick" which has appeared in a number of YouTube videos.


Key questions 

  • What assumptions have been made?
  • What is the area of each piece?
  • How would you convince someone else?


Possible support 

Explore the area calculation for a Parallelogram and then for a Trapezium by dissection.


Possible extension 

Do you recognise the numbers involved in this problem? Can you account for why this problem works with those numbers?


Try the problem Lying and Cheating.