Question 132577
Since total time is 8 hours:


If we let x be the current speed, the upstream speed is

20-x, downstream speed is 20+x

So  {{{75/(20-x)+75/(20+x)=8}}}

Multiply (20-x)(20+x) on both sides:

75(20+x)+75(20-x)=8(20-x)(20+x)

simplify it we get:

{{{3000=8(400-x^2)}}}

{{{3000 = 3200-8x^2}}}

That is {{{8x^2=3200-3000}}}
{{{8x^2=200}}}
{{{x^2=25}}}
x=5

So the answer is 5 miles/hr.