1. The problem statement, all variables and given/known data 2. Relevant equations g = GM / r^2, where g is the gravitational field strength, G is the gravitational constant, M is the mass of the attracting body, r is the radius of the attracting body. p = M / v, where p is density and v is the volume. Vs = 4/3 * pi * r^3, where Vs is the volume of a sphere, r is the radius of the sphere. 3. The attempt at a solution g = M / r^2 (as G is constant) rearraging p = M / v, M = pv pv / r^2 = g (p * 4/3 * pi * r^3) / r^2 = g (assuming the planet is perfectly spherical) p * 4/3 * pi * r = g density of Q is 1/2 that of P, radius is 2x that of P. 1/2 * 4/3 * pi * 2 = 4/3 * pi therefore, 4/3 * pi * 13.4 = 56.13Nkg^-1, however the answer is 13.4Nkg^-1? Can someone see where I've gone wrong?