Questions I Got Wrong

I got 47/50, here you can see my score.

Here is one of the questions I got wrong. Now I realize that X and Y can be run at the same time, and then Y finished in 50 seconds, so then Z can start running, and it will take 20 seconds after X finishes, so 80 seconds in total.

For this question, I realize now that A and D can communicate through E, so only one computer is necessary.

For this question, I now realize that the answer I chose begins at the incorrect index. A is correct because it starts at index 2 and ends at the last index.