Question

Of the 1,000 students in a local college, 420 own brand X mobile phones and 580 own brand Y mobile phones. Of these students, 80 own both brands of mobile phones. Find the probability that a student chosen at random has a brand X mobile phone given that he has a brand Y mobile phone.

Answer 1

Since we are given that the student has a brand Y phone, they are one of a sample of only 580 students.  Since 80 of those students also own a brand X (they own both) the probability that the random student is one of those is 50/580 = 5/58 = 8.62%