Hello friends, I need help with a geometry question...

http://s8.postimage.org/z1zbxoe39/Semicircle_Triangle.jpg this is the triangle.

Here is what we know:

1) PQ is the diameter of the semicircle.

2) OE equals Radius

3) PQ is parallel to BC

4) AD is the altitude to BC

5) BC = 20, AD = 15

Now, the first part of the question asks us to prove that ABC and APQ have similarity. This I managed to do, so treat it as data.

The second part is to find the radius, and this is where I'm stuck...

Plz help me!

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