1.25 g of a sample of $\text{Na}_2\text{CO}_3$ and $\text{Na}_2\text{SO}_4$ is dissolved in 250 mL solution. 25 mL of this solution neutralises 20 mL of $0.1\ \text{N}\ \text{H}_2\text{SO}_4$. The \% of $\text{Na}_2\text{CO}_3$ in this sample is