mate

3 men can unload a van in 20 minutes. how long it takes to do the same job with two men?\

{Google Translator}

If 3 men can unload a van in 20 minutes, then it means that 3 men can finish 1/20 of the job in 1 minute. Or 1 man can finish the job in: 3 x 20 =60 minutes.
So, if one man can finish 1/60 of the job in 1 minute, then 2 men can finish 2 x 1/60=1/30 of the job in 1 minute. Therefore, 2 men can unload the van in 30 minutes.

Here's another way to see this.......2 men are just 2/3rds the labor of 3 men.....therefore....it should take them 3/2 times as long to do the job.....so....

3/2  * 20    =  30 minutes