Planet X has the same mass as earth, but its radius is only half as big. How does g_x compare with our g?

PURPLE: g_x = g

PINK: g_x = 2g

YELLOW: g_x = 4g

GREEN: g_x = (1/2) g

BLUE: g_x = (1/4) g

Answer: YELLOW. Little g is given by G M(planet) / R(planet)^2. Since R is 1/2 as big, R^2 is 1/4 as big, and this is in the denominator. (Note that such a planet is VERY dense, I don't believe there are really any planets like this!)