There are several ways you could think it out. Here are two:

If 2 workers take 4 hours to process them, then 1 worker would take twice as 
long, or 8 hours.  Therefore it would take 5 workers only one-fifth as long,
and one-fifth of 8 is 8/5 or 1 3/5 hours or 1 hour and 36 minutes.

Or

If 2 workers take 4 hours to process them, then 10 workers could do that
in one-fifth the time or 4/5 of an hour, and only 5 workers would take twice
that long, or 8/5 or 1 3/5 hours or 1 hour and 36 minutes.

Edwin