What is the oxidation number of Mn in K_2MnO_4 ?

Let the oxidation number of $\mathrm{Mn}$ in $\mathrm{K_2MnO_4}$ be $x$.

The oxidation number of potassium ($\mathrm{K}$) is $+1$ and that of oxygen ($\mathrm{O}$) is $-2$.

Since the sum of oxidation numbers of all atoms in a neutral molecule is zero, we have:

$2(+1) + x + 4(-2) = 0$

$2 + x - 8 = 0$

$x - 6 = 0$

$x = +6$

The oxidation number of $\mathrm{Mn}$ in $\mathrm{K_2MnO_4}$ is $+6$.

Answer: $+6$