[corecase]

This is how i would do it:

int m = mark;
int d = daysLate;

if(m <= 20)
{
    System.out.println("Mark is: " + m);
}
else
{
    while(d >= 0 && m > 20)
    {
         m -= 5;
         d--;
    }
    System.out.println("Mark is: " + m);
}

I think it's a bit easier to understand than using a for loop with two variables inside. Just my opinion though! Hope it helps!