Word Problem Distance

Question

The illumination produced by a light source varies inversely as the square of the distance from the source.  The illumination of a light source at 5 m is 70 candela.  What is the illumination 12m from the source?

Answer 1

This is classic inverse proportionality: I = k/d² 

In problems of this type, the first sentence gives the formula, which I wrote above.

 

The second sentence gives you enough data to enable you to calculate the constant k, because the problem gives you specific data for I and d.  Solve for k using those givens.

 

Lastly, the third sentence gives you some data; using the k value just obtained, you can solve for the other value, in this case illumination I.