Physics density/moments question.

A rock balances a uniform 50g meter rule when it is hung the 10cm mark and the fulcrum at the 20cm mark. When it is lowered into a measuring cylinder the water level rises from 150cm^3 to 190cm^3. Calculate the density of the rock.

I understand that I have to work out the mass of the rock by using moments but I don't understand moments at all. Someone please help?

Moments is just that the sum of clockwise moments must be the sum of the anticlockwise moments

Imagine a seesaw where the balancing bit is directly in the centre and one person sits on either side of it.

So if I'm 50kg and I sit 2m away from the centre then my total moment is 50*2 = 100.

If there was someone who weighed 25kg, and we wanted the seesaw to be perfectly balanced, then they would need their distance from the centre, d, to be such that 25*d = 100, I.e. Sum of clockwise moments = sum of anti-clockwise moments.

Now the moments in this question are the ball or whatever it is (stupid TSR reply system) and the weight of the beam itself - as it's uniform, you assume it's at the centre.