<PRE><B>Question 257275</B>
John has a boat that will travel at the rate of 15 kph in still water.
 He can go upstram for 35 km in the same time it takes to go 140 km downstream.
 How fast is his boat traveling when he goes downstream?
:
Find the current first
Let c = speed of the current
then
(15-c) = speed upstream
and
(15+c) = speed downstream
:
Write a time equation; Time = dist/speed
:
Time down = time up
{{{140/((15+c))}}} = {{{35/((15-c))}}}
Cross multiply
35(15+c) = 140(15-c)
:
525 + 35c = 2100 - 140c
:
35c + 140c = 2100 - 525
:
175c = 1575
c = {{{1575/175}}}
c = 9 km/hr is the current
then
15 + 9 = 24 km/hr his speed downstream
;
:
Check solution by finding the times
140/24 = 5.83 hrs
35/6 = 5.833 hrs</PRE>