The number of equivalent $\mathrm{Cr}-\mathrm{O}$ bonds in ${\mathrm{CrO}}_4^{2-}$ is

Compound, ${\mathrm{CrO}}_4^{-2}$ has a tetrahedral structure and there are a total of $4\mathrm{Cr}-\mathrm{O}$ bonds in this structure. Although, two of them are single bonds and the other two are double bonds. Hence, it seems that not all Cr-O bonds are same.

But, in this case, two oxygen atoms are negatively charged and these negative charges are in conjugation with rest of O atoms which forms double bond with Cr. This indicates that in molecule, there is resonance due to which 4 Cr-O bonds acquires double bond character and becomes equivalent.

Therefore, all $4\mathrm{Cr}-\mathrm{O}$ bonds are equivalent in ${\mathrm{CrO}}_4^{-2}$.