Second warm-up: How can you achieve a 1/4 probability of getting seven good flares if the bad pile again contains three duds?Solution: Pick a pile at random to get six of the flares.

• 第二道暖身题：仍然假设坏盒里头有三枚哑弹，你如何用不低于1/4的机率，找到七枚好弹？