Convert numeric message from RSA ciphertext to alphabetic
The term you are looking for is Modular Arithmetic.

In the case of 1535, if it is indeed a combination of 3 values ranging 0 to 25, you do the following:

1535 mod 26 = 1
(1535 - 1) / 26 = 59
59 mod 26 = 7
(59 - 7) / 26 = 2

The set of values that generated 1535 is 2,7,1 (c,h,b?), which can easily be verified.