Question 1040157
<pre>
We find the LCM of 18 and 21
Line up the prime factorizations,
then bring them down to the bottom line
and multiply: 

 18 = 2×3×3
 21 =     3×7
-------------
LCM = 2×3×3×7 = 126

Answer = 126 minutes [which is 6 complete laps for 
                      the slower runner, since 6×21=126
                      and 7 complete laps for the faster 
                      runner, since 7×18 also = 126.

Edwin</pre>