Natural boron has two stable isotopes. Boron-10 has an atomic mass of 10.0129 u and boron-11 has…

Step 1. The equation. $10.811 = 10.0129\,x + 11.0093\,(1 - x)$.

Step 2. Solve. $11.0093 - 10.811 = 0.9964\,x$, so $x = 0.1983 / 0.9964 = 0.199$.

Step 3. Percentages. Boron-10 is $19.9\%$ and boron-11 is $100 - 19.9 = 80.1\%$.

Step 4. Mass of boron-10. The mass fraction is $\dfrac{0.1990 \times 10.0129}{10.811} = 0.1843$. In $250\ \text{g}$ there is $250 \times 0.1843 = 46.08\ \text{g}$, which is $46.1\ \text{g}$.

Source. Isotope masses are the values of the atomic mass evaluation (AME2020), and $10.811\ \text{u}$ is a standard atomic weight of boron.