Explanation:
as in the other problem, the table describes mutually exclusive attributes.
so, we can simply sum the numbers up to get the total number of students :
3+9+2+16 = 30 students.
so, when picking a random student, we have 30 possibilities.
now, how many of those 30 have the desired attributes (to have a brother AND a sister) ?
I see 3 in the table.
so, the probability to pick a student that has a brother and a sister is
3/30 = 1/10 = 0.1