



 
Hi Tom, I drew a view of the triangular prism with a ball inside, looking down from the top. I let r be the radius of the ball.
C is the centre of the circle and P is the midpoint of one of the sides. Since this is an equilateral triangle the symmetry gives us the measures of the angles. Thus triangle CPV is a 306090 triangle with the length of CP being r units. You can now use trigonometry to find the length of PV or perhaps you know the ratio of the sides of a 306090 triangle. Penny  


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