Out of 580 applicants to a graduate program at an Ivy League school, Marissa's GRE score was in the 50th percentile. How many applicants scored lower than she did?

50% because a percentile divides up the entire population into 100 groups and in this case she scored in the 50th percentile indicating that 50 groups are below and above her. so in this case 290 applicants (580*0.5) scored lower than her

well its groups so the fact in which we do not know where she performed in the group . such as the case that 580/100 is equal to roughly 5 kids per group. which means she could be 288-293. So in our case we just assumed that she is in the middle and that there are roughly 290 students below her.