Answer 1

A, B and C are the elements of a Dobereiner's triad. If the atomic masses of A and C are 7 and 39 respectively, then according to Dobereiner's triad law, the atomic mass of B is the average of the atomic masses of A and C.

So, the average of the atomic masses of A and C `=(7+39)/2= 46/2=23`
Therefore, the atomic mass of B is 23.