Is 243 a prime number

Answer 1

No. There are shortcuts to see if various numbers will go into it. To check for being able to divide by 3, you add all the digits together and see if 3 goes into the sum of all the digits. If the resulting number is still too big to work with, then repeat the process. So if you add 2 + 4 + 3, you get 9, and 3 goes into 9 (and thus 243 as well). Since this number can be evenly divided by 3 (81 times, by the way), it cannot be a Prime number.