175 of the 495 students who eat regularly in the cafeteria are 10th graders, because 175 is mentioned in the row "Yes" and in the column "10th" of the above table.

The probability is the number of favorable outcomes divided by the number of possible outcomes

P(10th|Yes)=# of favorable outcomes/# of possible outcomes=175/495=35/99 ~ 0.3535=35.35%