Posted by me on Wednesday, February 27, 2013 at 6:00pm.
Find the largest four digit number which has a total of exactly 3 factors. Assuming that 1 and the number itself are factors

maths help  Steve, Wednesday, February 27, 2013 at 6:13pm
you want the largest perfect square less than 10000. The three factors will be 1,√n, and n
Any number not a perfect square will have 4 or more factors (or just 2 factors if it is prime) 
maths help  Daniel, Monday, February 15, 2016 at 11:48am
The largest prime less than 100 is 97 so the largest four digit number with 3 factors is 97^2=9409.