Question 25
A box initially contains seven black balls and one white ball. Add a black ball every five seconds at 0:05, 0:10,..., 1:00, and add a white ball at 0:29. Starting at 0:00, draw one ball every 20 seconds, after any addition at that time. Find the probability of drawing a white ball within one minute.
Editorial interpretation: drawn balls are removed, not replaced. Draw uniformly at 0:00, 0:20, 0:40 and 1:00, including the endpoint. The target is at least one white ball.
Editorial interpretation: drawn balls are removed, not replaced. Draw uniformly at 0:00, 0:20, 0:40 and 1:00, including the endpoint. The target is at least one white ball.