The oxidation states of transition metal atoms in ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$, KMnO₄ and ${\mathrm{K}}_2{\mathrm{FeO}}_4$ are x, y and z respectively. The sum of x, y and z is ______ 

In ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$, oxidation state of Cr is +6. So, the value of x is 6. In ${\mathrm{KMnO}}_4$, oxidation state of Mn is +7. So, the value of y is 7. In ${\mathrm{K}}_2{\mathrm{FeO}}_4$, the oxidation state of Fe is +6. So, the value of z is 6. 

$x+y+z=6+7+6=19$