Since it takes 3 workers 1 hour to pack 6 boxes, it would take those 3 workers
6 times as long to pack 36 boxes, or 6 hours.  But 3 times as many workers (9
workers) could pack them in one-third the time, or 2 hours.

Or you can use the worker-time-job formula, which is:



where

W1 = the number of workers in the first situation.
T1 = the number of time units (hours in this case) in the first situation.
J1 = the number of jobs (boxes filled) in the first situation.

W2 = the number of workers in the second situation.
T2 = the number of time units (hours in this case) in the second situation.
J2 = the number of jobs (boxes filled) in the second situation.

W1 =  3             W2 = 9     
T1 =  1             T2 = the unknown quantity 
J1 =  6             J2 = 36







 reduces to  and  reduces to 



Cross-multiply

2T2 = 4

Divide both sides by 2

 T2 = 2

Answer: 2 hours.

Edwin