Your Number Was...
Why do this problem?
This problem provides a great opportunity to introduce the concept of representing operations on unknown numbers algebraically. It leads to work on inverse operations and solving simple linear equations. When first shown the interactivity, students may be amazed and astonished that it can always work out the starting number. The intention is to provoke their curiosity so they become
determined to explain the mystery.
Possible approach
One way to begin thinking about this is to collect together some outputs and their corresponding inputs, for example:
| Finishing Number | Starting Number |
| 11 | 5 |
| 17 | 8 |
| 3 | 1 |
| 23 | 11 |
| 14 | 6.5 |
- Represent the instructions as a function machine, and explore what happens when the functions are inverted.
- Represent the chosen number as $x$ and write an expression for the final number $y$ in terms of $x$. Then rearrange to get $x$ in terms of $y$.
- Discuss simplification to make the functions easier to invert (so representing it as $y=2x+1$ rather than $y=2(x+4)-7$.
At the end of the lesson, a selection of students can read out their instructions for the class to try, and then work out starting numbers from the final numbers.
In the "Settings" menu for the interactivity, you can edit the functions used by the machine. Click below to learn more.
You can enter your own sequence, separated by hyphens, using the code "a" for add, "s" for subtract, "m" for multiply and "d" for divide. The sequence a4-m2-s7 in the screenshot represents "Add 4, Multiply by 2, Subtract 7".
Once you have entered your chosen sequence, click to load it in the current window or a new resizable window.
Key questions
If you think of a number and add four, what do you need to do to get back to where you started?
If you think of a number, add four, and then double, what do you need to do to get back to where you started?
Is there a way of representing the "Think of a number" trick that helps you to get back to the starting number?
Possible support
In the problem Your Number Is..., students can explore visual and symbolic ways of representing number tricks.Possible extension
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