The intensity I of light from the light bulb varies inversely as the square of the distance D from the light bulb. Suppose I is 360 when the distance is 4. So the first thing we're going to do is I equal K / d ^2. So 360 goes in for the I and four goes in for the d ^2. So we're going to cross multiply 360 * 16 was 5760. When we substitute that in, we actually get our equation to be I equal 5760 / d ^2. So now the second part says how much farther would it be to a point where the intensity is 250 so I is going to be 250. Our D we're trying to find 5760 is in the numerator as our K. If we cross multiply and divide by the 2:50 we're going to have sqrt 5760 / 200 and 2005 or 250. I just dropped the zeros off because it makes it easier. Sqrt 576 is 24, sqrt 25 is five. Now it says how much farther would it be? So 24 fifths is how far it is now. But it was 4 before, so 24 fifths -20 fifths would give us 4/5.