An element A in a compound ABD has oxidation number ${\mathrm{A}}^{\mathrm{n}-}$. It is oxidised by ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$ in acid medium. In the experiment $1.68 \times {10}^{-3}$ moles of  ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7^{ }$ was used for $3.26 \times {10}^{-3}$ moles of ABD. The new oxidation number of A after oxidation is

Cr changes its oxidation state from +6 to +3 in acidic medium on oxidizing a compound. 

So, the net change in oxidation per molecule of K₂​Cr₂​O₇​ is +6 (2 Cr atoms per molecule). 

As 1 equivalent of K₂​Cr₂​O₇​ oxidizes two equivalents of ABD, then each A atom changes its oxidation state by 3.

So, the new oxidation state is 3−n.