Use a fraction to find the percent of the number 90% of 20

90%=90/100=9/10
9/10 of 20=9/10 times 20=180/10=18

the answer is 18

The circumference of the pond is key.

C=2pi*r, so the diameter is given, so it'd be the other circumference formula:

C=pi*d
.25*pi=0.785398163...
Or in terms of pi it'd be pi/4 (or .25pi)

Next, take half of the circumference.
0.39269908169...
This is how far your friend walked.

However, te question isn't how far your friend walked, it's how much further than you he walked, so

0.392699081698724
-.25
=0.142699081698...
Your friend walked 0.142699081698... more miles than you when going around the bridge.