The atomic masses of beryllium, magnesium and calcium are 9.0 u, 24.3 u and 40.1 u.

Step 1. Average. $\dfrac{9.0 + 40.1}{2} = 24.55\ \text{u}$.

Step 2. Difference. $24.55 - 24.3 = 0.25\ \text{u}$.

Step 3. As a percentage. $\dfrac{0.25}{24.3} \times 100 = 1.0\ \%$, which is within $2\ \%$.