Question 1168102
trip downstream at s+4 mph so 36/(s+4) hours
upstream at s-4 mph (s-4) so 36/(s-4)
we know that the second is (3/4 hour) more than the first
so 36/(s+4)+(3/4)=36/(s-4)
clear fractions
36(s-4)+(3/4)*(s^2-16)=36(s+4)
36s-144+(3/4)s^2-12=36s+144
(3/4)s^2-156=144
(3/4)s^2-300=0
s^2-400=0
s=20 mph only positive root.
24 mph downstream 1.5 hours to go 36 miles
16 mph upstream and takes 2.25 hours