Question 25
At 0:00 a box has six black balls and one white ball. Add one black ball every 10 seconds at 0:10, 0:20, …, 1:00, and one white ball at 0:29. Draw one ball at 0:00, 0:30 and 1:00, after the black-ball addition at that time. Find the probability of drawing a white ball within the minute. Editorial model: each draw is uniform among the balls currently present, and drawn balls are removed without replacement; the event is at least one white ball in the three draws.