(a)  When you perform the experiment under given condition (with replacement), you return the initial conditions 

     to the same state at every step, so the probability to get a red marble IS THE SAME in each sampling 

     and is equal to the ratio of the number of red marbles to the total number of samples


           P =  =  = .



(b)  In experiment (b), you have different initial condition at every step, since the number of red marbles is decreasing 

     and the total number of remaining marbles is decreasing, too.


     The probability of drawing a red marble at the sixth sampling is again the ratio of the number 

     of red marbles to the total number of marbles at this step, but these numbers are different at each step.


     At the 6-th step, you have only 10-5 = 5 red marbles in the box with the total number of marbles 20-5 = 15,


     so the probability to get a red marble at the 6-th step is


            P =  = .