Try to think the classification by Newlands and then think about the order of elements according to the increasing atomic masses.

Try to memorize the total number of elements discovered

Work out the last element which could be categorized accordinf to Newland's classification

Work out the average of the atomic masses two extreme elemnts and compare it to that of the middle element

Think about Dobrainer's triads and work out the base of classification

Think about the basis of classification of the triads given by Dobrainer

Think about the Newlands classification structure.

It is one of the multiples as it refers to Octaves

Think about the common property of the Dobrainer's Triads