Titration calculations

A student titrates 25.0 cm³ of sodium carbonate solution with 0.200 mol/dm³ hydrochloric acid using the reaction:

$\,\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{CO}_2 + \text{H}_2\text{O} \,$.

The average volume of acid used is 18.0 cm³. What is the concentration of the sodium carbonate solution in mol/dm³?