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Cancelling Fractions

Age 14 to 16
ShortChallenge Level Yellow starYellow starYellow star
Secondary curriculum
  • Problem
  • Solutions

Using factorials
Choose small numerators, say $2, 3, 4, 5, 6, 7$
$2\times3\times4\times5\times6 \times 7= m$ would give $(m+2)$ divisible by $2$,
                                    $(m+2) + 1$ divisible by $3$,
                                    $(m+2) + 2$ divisible by $4$,
                                                   .....
                                    $(m+2) + 5$ divisible by $7$

So $n=2$, and $m=2\times3\times4\times5\times6\times7+2$


Using smaller numbers
From the method above, we could have started from any number that was a multiple of $7$, $6$, $5,$ $4,$ $3$ and $2$ - so we could have started from the lowest common multiple of $7$, $6$, $5,$ $4,$ $3$ and $2,$ which is $7\times5\times4\times3= 420$. So, if $n=420+2=422$ and $m=2,$ the cancelling fractions are $$\frac{2}{422},\frac{3}{423},\frac{4}{424},\frac{5}{425},\frac{6}{426},\frac{7}{427}$$

Starting from $m=2$
Look for an $n$ corresponding to $m=2$. Then we require $n$ to have factor $2$, $n+1$ to have factor $3$ and so on. Hence $n$ cannot have any of the factors $3, 4, 5, 6$ or $7$. Since $n+3$ must be divisible by $5$, we must have $n$ ending in a $2$ or a $7$, but $n$ is even so it must end in a $2$.

In order for $n+2$ to be divisible by $4$, the number must be made up of an even number of tens with remainder 2. Since $n+1$ is divisible by $3$, we must also have the number of tens divisible by $3$.

The final condition is that our number which we have now determined to be of the form $n=60k+2$ is $5$ less than a multiple of $7$ ($n+4$ is automatically divisible by $6$). This is clearly true for $k = 7$, so $n = 422$ gives the fraction $$\frac{2}{422}$$ with the required property.


You can find more short problems, arranged by curriculum topic, in our short problems collection.

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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