Question 485629
it took 20 min to go up stream. 15 to come back. the speed off the current is 5 mph.
What is the speed of the boat?
:
We are dealing with mph therefore: 
Change 20 min to {{{1/3}}} hr
Change 15 min to {{{1/4}}} hr
:
let s = speed of boat in still water
then
(s-5) = effective speed up stream
and
(s+5) = effective speed down stream
:
Write a dist equation; dist = speed * time
:
up dist = down dist
{{{1/3}}}(s-5) = {{{1/4}}}(s+5)
multiply by 12 to get rid of those fractions, results
4(s-5) = 3(s+5)
4s - 20 = 3s + 15
4s - 3s = 15 + 20
s = 35 mph speed of boat in still water