Answer:
Step-by-step explanation:
<u><em>System A and System B are </em></u><u><em>not equivalent</em></u> !!!
Yes. Parenthesis first, then if any exponents, then multiplication or division, then addition and subtraction. PEMDAS.
The number of permutations of the 25 letters taken 2 at a time (with repetitions) is:

The number of permutations of the 9 digits taken 4 at a time (with repetitions) is:

Each permutation of letters can be taken with each permutation of digits, therefore the total number of possible passwords is: