In the mixture of  ${\mathrm{NaHCO}}_3$  and ${\mathrm{Na}}_2{\mathrm{CO}}_3$  the volume  of HCl required is x ml with phenolphthalein and further y ml is required with methyl orange indicator. Hence the volume of HCl for complete reaction of ${\mathrm{NaHCO}}_3$present in original mixture is