A grapefruit has a weight on earth of 4.9 newtons. what is grapefruit's mass

The force of gravity is the force with which massively large objects such as the earth attracts another object towards itself. All objects of the earth exert a gravity that is directed towards the center of the earth. Therefore, the force of gravity of the earth is equal to the mass of the object times acceleration due to gravity.

F = ma

4.9N = m (9.8 m/s^2), note that N (newtons) = kg-m/s^2

m = 0.5 kg

The mass of the object is 0.5kg.