Question 824303
The wall took a total of 6*12 = 72 man days to build.<P>
1 (the job) = x * 72 where x is the rate each man works.<P>
1/72 = x = the amount of the wall each man builds per day.<P>
1/72 * 12 = 12/72 of the wall built (by 12 men) each day.  We need a total of 72/72 (1) wall.<P>
72/12 = 6 days for the 12 men to build the wall.<P>
You can also look at it simple.  12 men complete the wall twice as fast as 6 men.  They complete the job in half the time.  So 12 men complete the wall in 12/2 = 6 days.