25ml of NaHCO3& Na2CO3 is titrated with N/20 HCl soln.In phenolphthalein indicator soln.needs 20.5mlof HCl.29.5ml is added.Find strength of Na2CO3...

Let's write the two steps of reaction of Na2CO3 with
HCl

Na2CO3 + HCl -> NaHCO3 +
NaCl

NaHCO3 + HCl -> H2O + CO2(g) +
NaCl

The volume of HCl used to titrate each 25mL of Na2CO3
is of 50mL.

Therefore, if you want to titrate 25mL of
Na2CO3, you'll have to use a volume of HCl which is twice then the volume of
Na2CO3.

If you have an amount of 20.5 mL of
HCl, then the amount of Na2CO3 is  of 20.5/2 = 10.25
mL.