Answer 1

1. You don't need the if else statement at at. 1 line is all you need after getting the number of passengers.
2. How do you expect this line to work;
int z=y-(x%y);
when you are getting the value for x after it and not before?
FYI, the r-value of that statement is all you need to output.

Answer 2

Koloroj

Why you are writing if else even you can get solution in few lines.

Just do

int seats = 50 - passengers % 50;

cout << seats;

Where passengers is input.