Average Rates Formula Question

<p>[2<em>Speed1</em>Speed2] / [speed1 + Speed2]</p>

<p>This formula for a question such as "Shelby went from her house to the mall at 7mph and came back the same way at 12mph. If the entire round trip took an hour, how far was the distance between the house and the mall?"</p>

<p>What is the background behind this formula? I can't seem to figure out where it came from T_T</p>

<p>It doesn't matter, it gives you the right answer</p>

<p>Sorry for resurrecting this thread but I was wondering this as well</p>

<p>Distance / Speed 1 + Distance / Speed 2 = Total time.</p>

<p>In this case, </p>

<p>D / 7 + D / 12 = 1</p>

<p>Therefore, D (the distance between the mall and the house), is equal to: I don’t know. You calculate it.</p>