To whom it may concern,  I have difficulty in getting the solution to the following question:  
 Find 5 numbers that have exactly 5 factors.  
 I got 16, 81 but couldn't find the rest. I believe that in order to have 5 factors, it has to be a square number. Isn't it true? I guess there may be a pattern to this. 
 Thanks for your help.
 Derek Yau.