Marginal probability

It answers a question about a single variable while ignoring the other one, which is why the totals live in the margins of the table. You divide a row or column total by the grand total, so the denominator is always the full sample size. Among 200 surveyed students, 80 are seniors, so $P(\text{senior}) = 80/200 = 0.40$. That value also equals the sum of the joint probabilities in the senior row, since those cells split the row total into pieces.