We know the center of gravity of semi circular arc is $\frac{2a}{\pi}$ (when its radius is a) measured from center of circle. How to deduce the center of gravity of a hollow semi-sphere from this fact.
I tried it by rotating semicircular arc around its own center(as shown in image) but then I found it hard to calculate the mass of the element unit( shaded area small mass). Please Help me. Thanks.
You can't use that fact in that way to solve that problem. The shaded element in the diagram has varying width, so its centre of mass isn't at the centre of mass of the semi-circular arcs that bound it, but further from the centre, closer to the part with greater width.