Question 1146051
d, distance between A and B
d/3, distance between B and C

<pre>

            SPEED       TIME           DISTANCE

A to B       4-k          1                 d

B to C       4+k         {{{1/5}}}             {{{d/3}}}
</pre>


{{{system(4-k=d,(1/5)(4+k)=d/3)}}}

-

{{{(3/5)(4+k)=d}}}
-
{{{4-k=(3/5)(4+k)}}}

{{{20-5k=3*4+3k}}}

{{{20-12=8k}}}

{{{8k=8}}}

{{{highlight(k=1)}}}