Suppose the speed that they paddled was x mi/h
speed of current was 3 mi/h
speed of moving upstream
(x-3) mi/h
speed of moving downstream
(x+3) mi/h
one way distance=24/2=12 miles
time taken to move up stream:
12/(x-3) hr
time taken to move downstream
12/(x+3) hr
total time they paddled:
(8:30 a.m -4:00 p.m)-(1:30)
=7.5-1.5
=7 hours
thus
12/(x-3)+12/(x+3)=7
solving for x we get:
=24x/(x-3)(x+3)
24x/(x^2-9)=7
thus
x=5.1695~5.2 hours
x=5.2 mi/hr
thus
time taken to paddle downstream:
12/(5.2-3)
=5 5/11 hours
=5 3/11 hours
=5 hours 27 minutes
~5 hours 25 minutes