Question

the density of a gas A is three the gas B if molecular weight of A is M then molecular weight of B is

Answer 1

let us consider density of A be x and density of B be y.
given molecular wt of A=M
given x=3y
molecular wt of A by volume of A=3xmolecular wt of B by volume of B
but if we consider 1 mole of gases they occupy 22.4lit volume for all gases
from above we can say that 
MOLECULAR WT OF B = ONE THRD OF MOLECULAR WT OF A
                                    =1by 3 x M