If oxidation state of $\mathrm{Cr}$  in ${\mathrm{CrO}}_5$  is  $X$, oxidation state of $\mathrm{Cr}$  in  ${\mathrm{K}}_3{\mathrm{CrO}}_8$  is  $Y,$ then $\mathrm{X}-\mathrm{Y}$  is equal to:

$x=6 y=5 x-y=1$

In ${\mathrm{CrO}}_5$ four oxygen atoms are in peroxy linkage and in ${\mathrm{K}}_3{\mathrm{CrO}}_8$ all oxygens are in peroxy linkage