Question

My last question of the night..please help

The intensity I of light bulb varies inversely as the square of the distance d from the light bulb. Suppose I is 250 w/m^2 when the distance is 5 m. How much farther would it be to a point where the intensity is 40 w/m^2?

Answer 1

250 : 5
1: 1250
x : 40
x = 31.25m

Answer 2

I1= 250
I 2 = 40

D1= 5M
D2=?
\[I1\div I2\]=\[D2 ^{2} \div D1^{2}\]
\[250 \div 40 =D2^{2}\div25\]
\[D2 =12.5 M\]