Question 1045424
RT=D is Travel Rates Rule for constant rate R.

<pre>
                  RATE      TIME       DISTANCE
PERSON            {{{r[p]}}}      t
FRIEND            {{{r[f]}}}      t
TOTAL                                    d
</pre>

"PERSON" is referenced as traveling or traveled "from home".  The number for this will be {{{r[p]*t}}}.


<pre>
                  RATE      TIME       DISTANCE
PERSON            {{{r[p]}}}      t          {{{r[p]*t}}}
FRIEND            {{{r[f]}}}      t          {{{r[f]*t}}}
TOTAL                                    d
</pre>

Description established {{{system(r[p]=knownPersonRate,r[f]=knownFriendRate,d=knownDistanceBetweenPlaces,t=UnknownTime)}}}


{{{r[p]t+r[f]t=d}}}
{{{t(r[p]+r[f])=d}}}
{{{highlight(t=d/(r[p]+r[f]))}}}



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Practical thinking is to use that both people starting at same time going toward each other decrease in distance at the SUM of their two rates.  Calling this sum S, travel time t, known distance D, you can simply have {{{S*t=D}}} and then {{{t=D/S}}}.
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What this means for the first part of the example is time is {{{12/(4+11)}}} hours, or {{{12/15=(3*4)/(3*5)=highlight((4/5)hour=48*minutes)}}}.