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

this is a conditional probability. since we are given that the student has a brand Y mobile phone, the denominator of our probability is only the students who have brand Y

we want to know the probability that they'll have a brand X phone as well

so calculate: (people who have brand X and Y) / (people who have brand Y)