It rains 3 days a week. When it rains there is Thunderstorm.The probability of Thunderstorm falling on the ground is 1/42000.A Golfer plays golf on Saturday and Sunday. What is the probability of Thunderstorm falling on that Golfer?

The question can be done with help of Combination.

Total possibiliteis of days on which rain occurs = ⁷C₃

Total possiblities containing Sunday or Saturday as one of the days = ⁷C₃-⁵C₃

Probiabiltiy that the thunderstorm occurs on the Sundays and Saturdays = (⁷C₃-⁵C₃)/⁷C₃

Probablitlity tha thuder storms occur on that day = (⁷C₃-⁵C₃)/⁷C_{3 * 1/42000= 25/35*1/42000 = 5/294000}