An impure sample of boric acid of mass $0.640\ \text{g}$ is dissolved in water, mannitol is added, and the mixture is titrated as a monobasic acid with sodium hydroxide. It needs $38.0\ \text{mL}$ of solution of concentration $0.250\ \text{mol/L}$. What is the percentage of boric acid in the sample? Take $M = 61.83\ \text{g/mol}$.