Please help me, to solve this C# 5/2 = 2?

you can use / and Ceiling ..

your problem is
x = 5/2
x is allready 2, not 2.5 as you may suggest. You need to look into that kind of problems with the debugger. to see if the code does what you want it to do

in that case / on int numbers, gives you back .. int numbers, not double as you intend

x = (double)5/2
x will be 2.5 as you use / on a double value

now Math.Ceiling and Math.Floor will work as expected

just use int

int down= 5/2; // down=2
int mod = 5%2; // mod =1

int down= 6/2; // down=3
int mod = 6%2; // mod =0

float a = 5f; //decimal a = 5m; //int a1 = 5;
float b = 2f; //decimal b = 2m; //int b1 = 2:
double c = a % b;
double d = a / b;// duoble d = (double) a1 / b1;


Console.WriteLine($"divide {d} {c}");
Console.Read();