For example, when entering the number 13, it gives the answer 3.0

Answer 1, Authority 100%

Because B – int and 4 too int
Row Double C = b / 4; works like this:

double c = (double) (b / 4);

first occurs integer division, and then bringing to double ;

To the division was not integer Write:

double c = b / 4.0;

In this case, b will be Double everything will be correct.