You will have to do a fair bit of trial and error to work out prime numbers if you are not familiar with them. For example ~ Is 17 a prime number? Try 2x8=16, 2x9=18...it is not divisible by 2. Next try ~ 3x5=15 ~ 3x6=18 ~ nor 3. Next try 5x3=15 ~ 5x4=20. ~ Next try 7x2=14 ~ 7x3=21. Next is 11...and that does not work either.So 17 is a prime number. All this is much easier if you know your times-tables well