Gravity

Hey. I've just come back from an interview, and the interviewer asked me to go home and think about this.

We were discussing Newton's theory of gravitation. If and apple is placed somewhere above the Earth, you can measure the force using

F=G(M1M2)/r^2
Where F=Force of gravity,
M1 = Mass of Earth
M2 = Mass of apple
r= distance between he centre of the two objects

He then asked me how can we find G, we don't know M1 or r, but we can find M2 using a weighing scale, and F using a weighing scale. With this information only, how can we find G? Not necessarily an exact value, but how can we calculate it or what experiment can be done to find it?

Thanks

@Kyx