In the balanced chemical reaction,

${\mathrm{IO}}_3^{-} + {\mathrm{aI}}^{-} + {\mathrm{bH}}^{+} \to {\mathrm{CH}}_2\mathrm{O} + {\mathrm{dI}}_2$

a, b, c and d respectively correspond to

${\mathrm{IO}}_3^{-} + {\mathrm{aI}}^{-} + {\mathrm{bH}}^{+} \to {\mathrm{CH}}_2\mathrm{O} + {\mathrm{dI}}_2$

    $$\begin{gathered} \left(2\right) \Rightarrow 2{\mathrm{IO}}_3^{-}+10{\mathrm{e}}^{-}+12{\mathrm{H}}^{+}\to {\mathrm{I}}_2 + 6{\mathrm{H}}_2\mathrm{O} \\ \underline{\left(1\right)\times 5\Rightarrow 10{\mathrm{I}}^{-}\to 5{\mathrm{I}}_2 +10{\mathrm{e}}^{-} } \end{gathered}$$

 $\mathrm{On} \mathrm{addition}: 2{\mathrm{IO}}_3^{-}+10{\mathrm{I}}^{-}+12{\mathrm{H}}^{+}\to 6{\mathrm{I}}_2 + 6{\mathrm{H}}_2\mathrm{O}$

                        ${\mathrm{IO}}_3^{-}+5{\mathrm{I}}^{-}+6{\mathrm{H}}^{+}\to 3{\mathrm{I}}_2+3{\mathrm{H}}_2\mathrm{O}$

On comparing, a = 5, b = 6, c = 3, d = 3