.
When this problem first came to the forum
https://www.algebra.com/algebra/homework/Probability-and-statistics/Probability-and-statistics.faq.question.1209432.html
I was very happy. "Look, I thought, what a wonderful problem. This student is seriously interested
in Math and in solving advanced probability problems. He deserves my help and support."
This thought gave me enthusiasm, and I wrote a solution for part (a) (see the link above), and the next day
I wrote a solution for part (c) (see the same link above).
I thought that for an advanced student (and only an advanced student could bring such a task) this is enough
to complete the remaining parts (b) and (d) on his own, based on these solutions (a) and (c) of mine.
I expected to get from you something like this:
Dear tutor @ikleyn, thank you for presenting me the method, the idea and the technique
of solving to parts (a) and (c). Thanks to your post, I am now fully equipped to solve
the remaining parts (b) and (d) on my own, following the path paved by you for me.
I will apply all my efforts to do it and to prove that I correctly absorbed the ideas
and the technique from your post. Please allow me to post my future solution to you
for your review. Thanks again.
But instead of this, you start beating a big drum and write that parts (b) and (d) remained unsolved.
And in addition, you request from me to post you the missing solutions as soon as possible.
Thus, it turned out that you are not an advanced student at all, but are just a routine collector
of other people's solutions and are not going to do anything (no one single step)
to solve the remaining parts on your own. Although the way was paved and opened for you.
Alas, I overestimated you.
At this forum, my task and my goal is to teach incoming students, but not to write texts for them
(or instead of them) and/or not to make their job/(their wok) for them or instead of them.
With this, I say goodbye. All the best.
Now, as we established that your function at this forum is not studying, but collecting
solutions produced by other people, may I ask you, for what project do you really work ?
Do you collect solutions for Artificial Intelligence ?
Or may be, you collect solutions to fill somebodies' educational web-site ?
Or to create and fulfill an answers book, paper or electronic, for somebodies' textbook ?
Or, perhaps, these three separate activities are merged in one project ?
I am asking, because I do not see any other motivation for a person,
whose function is collecting of solutions created by other people.