so use pythagorean theorem where
a^2+b^2=c^2
a and b are legs
c=hypotonuse
in a right triangle
90 degrees=right riangle
so we have 20 and 15
these are the legs
subsitute
20^2+15^2=c^2
400+225=c^2
add
625=c^2
square root both sides
25=c
new pipe=25
how much shorter
old=20+15=35
how much shorter is 25 than 35
35-25=10
answer is 10 miles