CPM Homework Help : CALC Problem 11-130

Multiple Choice: The luminous intensity $E$ of a light bulb, measured in lumens per ft^{$^{2}$}, varies inversely as the square of the distance s from the bulb. $E = 5.2$ lumens per ft^{$^{2}$} when $s = 5$ ft for a $100$ watt bulb. If you are moving away from a $100$ watt bulb at a speed of $2$ feet per second and you are $3$ feet from the bulb, the luminous intensity is changing at the rate of:

$- \frac { 520 } { 27 }$ lumens per ft $^2$

$- \frac { 260 } { 27 }$ lumens per ft $^2$

$- \frac { 130 } { 27 }$ lumens per ft $^2$

$- \frac { 130 } { 9 }$ lumens per ft $^2$

$- \frac { 260 } { 9 }$ lumens per ft $^2$

Hint:

$5.2=\frac{k}{5^2}\text{ }k=?$
When $s = 3$ , $E =$ ?
Substitute in the known values and solve for $E^\prime$ .