Question

please tell question number 24

Answer 1

Mass of the third element c is 39u

Answer 2

Here is ur answer user....

according to Dobereiner's traid the average of mass if first and last element is equal to the middle element .

then,

suppose the mass of third element is = x

and now...
x+7/2=23
=x+7=46
=x=46-7
=x=39


So the mass of element is 39u.