Suppose the radius of planet A= RA And radius of planet B = RB And
Mass of planet A = MA, then find mass of planet B = MB.
Here, RA= RB/2, gB= ½ gA
Then, g= GM / R2 And gA = GMA /R2A
Therefore, gB = G MB/ R2B
Now, MB/ R2-B = ½ [GMA /(RB/2)2]
MB/ R2B= ½ [4 (GMA /(RB)2)]
So, MB= 2MA