$\frac{n (n - 1)}{2} = 10$
How to rearrange equation for finding value of $n$ .

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Solution

$\frac{n (n - 1)}{2} = 10$
$\Rightarrow n (n - 1) = 10 \times 2$
$\Rightarrow n^{2} - n - 20$
$\Rightarrow n^{2} - 5 n + 4 n - 20$
$\Rightarrow (n - 5) (n + 4) = 0$
$∴ n = 5, - 4$

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