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Black and White Socks

Age 14 to 16 Short Challenge Level:

Answer: 5


There are $20+n$ socks in the drawer altogether, so $\dfrac{n}{20+n}=\dfrac{1}{n}$.

$$\begin{split}&\frac{n}{20+n}=\frac{1}{n}\\
&\Rightarrow n=\frac{1(20+n)}{n}\\
&\Rightarrow n^2=20+n\\
&\Rightarrow n^2-n-20=0\\
&\Rightarrow (n+4)(n-5)=0\\ 
&\Rightarrow n+4=0\hspace{3mm}\text{or}\hspace{3mm} n-5=0\\&\Rightarrow n=-4\hspace{7mm}\text{or}\hspace{3mm}n=5\end{split}$$ $n$ must be positive as there cannot be a negative number of white socks, therefore there are $5$ white socks in the drawer.

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