In a first order reaction the concentration of the reactant falls from 0.50 mol/L to 0.32 mol/L in…

Step 1. The rate constant. $k = \dfrac{1}{15\ \text{min}}\ln\dfrac{0.50}{0.32} = \dfrac{0.446}{15\ \text{min}} = 0.0298\ \text{min}^{-1}$.

Step 2. Another 15 min. The same time brings the same factor, $\dfrac{0.32}{0.50} = 0.64$. So $0.32\ \text{mol/L} \times 0.64 = 0.205\ \text{mol/L}$.