In total school choir has 80 students, with 36 of them being in seventh grade. also, 40 students in the choir are girls where 18 of them are in seventh grade. what is the probability that a student picked at random from the choir is either a girl or is in seventh grade?

The sample space in this problem is 80 students. we are asked for  the probability that a student picked at random from the choir is either a girl or is in seventh grade. the number of girls is 40 and 18 students are in seventh grade. The probability hence is (40+18)/80 equal to 0.725

The density of the cube is 40