Question 1041826
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<pre>
{{{75/5}}} =  5 = u - v,    
{{{99/9}}} = 11 = u + v,

where {{{75/5}}} is the boy's speed relative to the floor when moving against . . .
{{{99/9}}} is the boy's speed relative to the floor when moving with . . .

u is the boy's speed relative to the moving sidewalk, v is the speed of sidewalk relative to the floor.

S, you have this two basic equations:

u - v =  5,   (1)   and
u + v = 11.   (2)

Add eqns(1) and (2) (both sides). You will get 

2u = 5 + 11 = 16  --->  u = {{{16/2}}} = 8 {{{ft/s}}} - speed of the boy rel. to the sidewalk.

The v = u - 5 = 8 - 5 = 3 {{{ft/s}}} is the speed of sidewalk rel. to the floor.

<U>Answer</U>.  8 {{{ft/s}}} and 3 {{{ft/s}}}.
</pre>

For details see these lessons

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/word/travel/Wind-and-Current-problems.lesson>Wind and Current problems</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/word/travel/More-problems-on-upstream-and-downstream-round-trips.lesson>More problems on upstream and downstream round trips</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/word/travel/Wind-and-Current-problems-solvable-by-quadratic-equations.lesson>Wind and Current problems solvable by quadratic equations</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/word/travel/Selected-problems-from-the-archive-on-a-boat-floating-Upstream-and-Downstream.lesson>Selected problems from the archive on the boat floating Upstream and Downstream</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/word/travel/Selected-problems-from-the-archive-on-a-plane-flying-with-and-against-the-wind.lesson>Selected problems from the archive on a plane flying with and against the wind</A> 

in this site.