Calculus: The Intensity of Light Change Question?

1. The intensity of light at a distance r from a source is given by L = I*r^(-2) where I is the illumination at the source. Start with the values I = 20, r = 60. Suppose we increase the distance by 3 and the illumination by 4.

By how much does the intensity of light change?

dL =?

Here is what i got: Both answers i got are wrong?? Can someone please explain. Thanks

1st attempt:

L = 20(60^(-2)) = 1/180

L = 24(63^(-2)) = 8/1323

(1/180)-(8/1323)=dl= **-4.91307634E-4. **

2nd attempt:

L = 20*60^(-2) = 1/180

L = 23*64^(-2) = 23/4096

(1/180)-(23/4096)=dL= **-5.96788194E-5**