Is 243 a prime number

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.