if 8 men can plough a field in 9 hours, how long would it take 18 men at the same rate?

if 8 men can plough a field in 9 hours, how long would it take 18 men at the same rate?
 

18 men can plow the field 18/8 times faster than the 8 men.
 
so it would take them 8/18 as long.  8/18 times 9 = 4, so the answer is 4 hours.