You have that the code contains three letters followed by one digit.
In order to determine how many codes are possible you multiply the number of possible combination of letters by the number of possibilities for one digit.
The number of combinatins for three letter is calculate as follow:
number of available letters code x number of letter in the alphabet
= 3 x 26
= 78
The numbers of combination of the final digit is 10, due to you take into account numbers from 0 to 9.
Finally, the number of possible codes are the product between the last two result:
78 x 10 = 780
Hence, there are 780 possiblities for the combination of the codes assigned to each student.
