Question 337204
No idea at all? 
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The intensity ({{{I}}}) of light from a light bulb varies inversely as the square of the distance ({{{d}}}) from the light bulb.
{{{I=k/d^2}}}
{{{640=k/(4)^2}}}
Solve for {{{k}}}.
Now you have {{{k}}}, find {{{d2}}} when {{{I=250}}}.
Then find the difference between {{{d2}}} and {{{4}}} m.
{{{d=d2-4}}}