Question 1200888
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the intensity of the radiation used to treat a tumor varies inversely as the square of the distance from that emits the radiation. 
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d, distance
I, intensity
k, variation constant


{{{I=k/d^2}}}


Your given values for the question:
{{{I=100}}}
{{{d=2.5}}}
k, you find this value from the formula.  Then use it to find some unknown d for the desired {{{I=39.0625}}}.