What is the difference between a closed and an open primary .

Asociologist chose 300 students at random from each of two schools and asked each student how many siblings he or she has . the result are shown in the table below. there are a total of 2,400 students at lincoln school and 3,300 students at washington school .what is the median number of siblings for all the students surveyed ? a)0b)1c)2d)3