



 
Hi Mike, From the diagrams in Stephens solution you can see that the radius R of the outside circle is the radius r of the inside circle plus w. That is
Using the symmetry of a circle he also shows that r/R is 911/1728. Hence
In Phil's problem w = 282 mm so
At this point multiply each side by (r + 282) to obtain
or
Now multiply each side by 1728 and the equation becomes
Thus
or
So
and finally
I hope this helps,  


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