Maths seems to have changed a lot since I used to teach it I can't make any sense of it at all!
From a strictly scientific standpoint, the original statement is incorrect anyway, since grams actually measure mass, not weight. If the question had started with 'the mass of a cube is 87g' the rest of it would make sense, since the minimum possible weight would be zero (in space) and the maximum weight would effectively be infinite (at the centre of a black hole), but I'd have expected such a question to have been set by his physics teacher, rather than by his maths teacher!
Chris