$$2\frac{n}{2}-\frac{3n}{4}+\frac{5n}{6}=21$$
$n = \frac{252}{13} = 19\frac{5}{13} \approx 19.384615385$
$$12n-3\times 3n+2\times 5n=252$$
$$12n-9n+2\times 5n=252$$
$$3n+2\times 5n=252$$
$$3n+10n=252$$
$$13n=252$$
$$n=\frac{252}{13}$$
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