Explanation:
ah, here you go.
come on, now. you know this now ...
again, mutually exclusive attributes in the table (no overlap here) allow us to add the numbers for a total :
4+2+12+10 = 28 students in total.
how many of those 28 fulfill our desired criteria (has a brother) ?
well, it is the sum of the column "has a brother", as it does not matter, if there is also a sister or not.
so, it is 4 + 12 = 16
therefore, the probability to pick a student randomly, who has a brother is
16/28 = 4/7 = 0.571428571...