Uniform semicircular lamina question

[brandon26]

Can someone please explain to me how to start off this question?

A uniform semicircular lamina has mass M. A is the midpoint of the diameter and B is on the circumference at the other end of the axis of symetry. A particle of mass m is attached to the lamina at B. The centre of mass of the loaded lamina is at the midpoint of AB. Find in terms of pie, the ration M:m .

Some one please help quickly

 

[HallsofIvy]

I suspect it is supposed to be in terms of pi, not pie!

1) Find the center of mass of the uniform lamina: it will be, of course, on the line AB. I don't know whether you are given a formula for that or if you are expected to do the integration to it.
2) The center of mass of two objects lies on the line between then dividing the line into segments or ratio m/(m+M) and M/(m+M). You are told that that is the midpoint of the radius of the circle. You can calculate ratio M:m from that.