consider a planet in some solar system which has a mass double that of earth and density equal to the average density of earth an object weighing w on earth will weigh

First of all, Mass of the planet Mp will become two times Mass of earth M. Since density is given to same you go with the formula M/R^3 and you compare Mass upon radius cube of Earth with the planet. Accordingly substitute the things and get the answer.. W = GmM/R^2 (the values on earth);'' now on the planet M_p = 2M and density is same hence M_p/(R_p)^3 = M/R^3 R_p = (cube root 2) * R Thus the W_p = [2/(cube root 4)]*W = (cuberoot 2)

You need to practice a lot for such types of numericals.. You can refer to some books like Marvel or USS and thus increase your knowledge and skill-set as well.

Best wishes. Thankyou.