I made an illustration of your problem Bertie.
Since the radius of the first circle from center to the common point is 15 and the radius of the second circle to the common point is 20 the distance between the two centers had to be $${\sqrt{{{\mathtt{15}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{20}}}^{{\mathtt{2}}}}} = {\mathtt{25}}$$
I'm not sure about how I can solve your question though
Since you said no calculus, I can hardly believe Melody's answer is correct 
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Sorry Melody 
