Conditional distribution

You restrict attention to one group, then re-divide by that group's total rather than the grand total, so the percentages inside the group add to 1. That re-dividing is the whole difference from a marginal distribution, which uses the grand total and describes everybody. Of 200 surveyed students, 80 are seniors and 45 of those seniors drive to school, so the conditional distribution of driving given senior is $45/80 = 0.5625$ driving and $35/80 = 0.4375$ not driving. Comparing the conditional distributions across rows is how you judge whether the two variables are associated.