Is 31 prime?  It is prime if it doesn't have any even divisors other than itself and 1.
Note: if 31 is not prime, then it has a divisor at most sqrt(31) = 5.something (because 6*6 = 36).

31/2 = 15, remainder 1
31/3 = 10, remainder 1
31/4 = 7, remainder 3
31/5 = 6, remainder 1
31/6 = 5, remainder 1

No even divisors.  31 is prime.


Is 33 prime?

33/2 = 16, remainder 1
33/3 = 11, remainder 0
33/4
33/5
33/6

33 = 3 * 11, so 33 is not prime.


Convert 1100 1100 to hexadecimal.
         CC

Convert 1100 1100 to decimal.
  128,64,32,16, 8,4,2,1
  128 + 64 + 8 + 4 = ...


Encrypt good with ceasar cipher, password = 4.

01                       25
abcdefghijklmnopqrstuvwxyz
      |---|

kssh

Decrypt qepq, it was encrypted by the password 23.
This means each letter was shifted right by 23.
Or, same as shifting left by 3.

To decrypt, do the opposite of encrypting.  Go left by 23, or go right by 3.

thst.




Selection sort on 3, 1, 7, 2, 8, 0, 4
This is what the list looks like as we go through the algorithm ...

0, 1, 7, 2, 8, 3, 4
0, 1, 7, 2, 8, 3, 4
0, 1, 2, 7, 8, 3, 4
0, 1, 2, 3, 8, 7, 4
0, 1, 2, 3, 4, 7, 8
0, 1, 2, 3, 4, 7, 8