Titration calculations

A student titrates 25.0 cm³ of 0.100 mol/dm³ sulfuric acid, H₂SO₄, with sodium hydroxide solution. The average volume of NaOH used is 50.0 cm³. The reaction is $\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$. What is the concentration of the sodium hydroxide solution in mol/dm³?