A, B, and C are the elements of Döbereiner’S Triads. If the atomic mass of element A is 9 and that of element C is 39, what is the atomic mass of element B?

Döbereiner’S Triads are groups consisting of three elements (triads) that have similar chemical and physical properties. Dobereiner called this grouping method the law of triads.

In every triad, the atomic weight of the middle element was half of the sum of the atomic weight of the other two elements. Properties of the middle element were also in the halfway of both the elements.

We can see that the triad that we have here is A, B, C. Here, B is the middle element. So, according to the law of triads, the mean of the atomic masses of A and C is expected to be equal to the atomic mass of B.
$\mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{B}=\frac{\mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{A}+\mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{C}}{2}$
$⟹ \mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{B}=\mathrm{ }\frac{9+39}{2}$
$\mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{B}=\mathrm{ }\frac{48}{2}$
$\mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{B}=24$

The atomic mass of element B is 24.