Please help me with this riddle

DeMar is a strange little planet. Its gravitational acceleration at its surface is exactly 10.0 metres per second per second, and its diameter is exactly 2100 kilometres.

Also, a completely unrelated fact, Skeiths are able to consume about 0.4 kg of pretty much anything they want to eat, every minute, nonstop.

Assuming that the density of the planet is uniform, and that orbiting bodies don't significantly affect the planet's gravity, how many years will it take one million Skeiths to consume one cubic kilometre of DeMar? Please round up to the nearest year.

Please help me

Hi,
this is not a riddle, it's a physics question.
If the density of the planet is uniform and nothing else affects the planet's gravity, then you just need to know how much a cubic kilometer weighs (hmm, or rather, what's its mass).
You have to find the mass of the planet (considering that formula F = G (m1 * m2)/r^2 ---> [link], you should find that Acceleration = G * MassPlanet / r^2), then after finding the volume you can find the mass of a cubic kilometer. Then after some other calculations you should get your result.