Suppose atomic masses of P and Q are X and Y respectively.
Molar mass of P2Q3=2X+3Y
and Molar mass of PQ2=X+2Y
Now Mole=massmolarmass
∴2X+3Y=15.9/0.15....(i)
and X+2Y=9.3/0.15....(ii)
Solving these equations, we get X=2.6gmol−1,Y=18gmol−1
So, atomic mass of Q is 18.