What was the Mendeleev’s basis for the classification of elements ?

A, B and C are the elements of a Dobereiner’s triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?

X and Y are the two elements having similar properties which obey Newland’s law of octaves. How many elements are there in-between X and Y ?

In the classification of the then known elements, Mendeleev was guided by two factors. What are those two factors ?

Name two elements whose properties were predicted on the basis of their positions in Mendeleev’s periodic table.