A charged sphere X is supported on an insulating stand. A second charged sphere Y is suspended by an insulating thread so that sphere Y is in equilibrium at the position shown in the figure below.
The charge on sphere X is 
and the charge on sphere Y is 
. Assume that the spheres behave as point charges. The length of the thread is 
and the centres of sphere X and sphere Y are separated horizontally by a distance of 
. Determine the mass of sphere Y.
.
