I cannot stress enough the importance of modular arithmetic - the math where we divide and focus only on remainders

Take any number N (preferably a prime - let’s use N= 5)

(Here = is congruence )

1(mod5) = 1

2(mod5) = 2

3(mod5) = 3

4(mod5) = 4

5(mod5) = 0

6(mod5) = 1

7(mod5) = 2

8(mod5) = 3

9(mod5) = 4

10(mod5) = 0

You seeing it?

3483638(mod5) = 3

Being good at this is SO IMPORTANT to learning cryptography. You have to be just as comfortable with these operations as nerds are with the other usual ones (addition, multiplication, etc)