<PRE><B>Question 471491</B>
A motorboat maintained a constant speed of 11 miles per hour relative to water in Going 30 miles upstream and then returning.
 The total time for the trip was 5.5 hours.
:
Let c = speed of the current
then
(11+c) = speed downstream relative to land
and
(11-c) = speed upstream relative to the land
:
Write a time equation: time = dist/speed
:
time upstream + time downstream = 5.5 hrs
{{{30/((11-c))}}} + {{{30/((11+c))}}} = 5.5 
:
Multiply by (11-c)(11+c), results:
30(11+c) + 30(11-c) = 5.5(11+c)(11-c)
:
330 + 30c + 330 - 30c = 5.5(121 - 11c + 11c - c^2)
:
660 = 5.5(121 - c^2)
:
660 = 665.5 - 5.5c^2
:
5.5c^2 = 665.5 - 660
:
5.5c^2 = 5.5
Divide both sides by 5.5
c^2 = 1
c = 1 mph is the current
:
:
See if this is true; find the times both ways
downstream: 30/12 = 2.5 hrs
up-stream:  30/10 = 3.0 hrs
----------------
total time: 5.5 hrs</PRE>