



 
Hi, In my diagram $CA$ and $BC$ are both radii of the hemisphere and hence are of the same length, call it $R$ cm. Triangle $ABC$ is an isosceles right triangle with hypotenuse of length 20 cm so Pythagoras Theorem will help you find the value of $R.$ Now use the procedure in Harley's response to an earlier question to complete the solution. Penny  


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