<p>Xiggi’s formula is what is more commonly know as the formula for the harmonic mean. </p>
<p>A common mistake is to take the “average” of two rates by using the arithmetic mean. But for rates the harmonic mean should be used.</p>
<p>Average rate = 2(rate1)(rate2)/(rate1 + rate2)</p>
<p>Note that this formula works only when the two distances are the same. Here is a sample problem where Xiggi’s formula can be used to get the answer very quickly:</p>
<p>An elephant traveled 7 miles at an average rate of 4 miles per hour and then traveled the next 7 miles at an average rate of 1 mile per hour. What was the average speed, in miles per hour, of the elephant for the 14 miles? </p>