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.