Samantha is preparing to do a survey of the language classes taken by ninth and tenth graders at her high school. Each student in ninth and tenth grade takes one language class. From school records, she knows there are 158 students in the ninth grade, as shown in the table. Language
Grade French Mandarin Spanish Total
9 r s t 158
10 x y z
Total

Enter an expression to represent the conditional relative frequency that a student takes Mandarin, given that the student is a tenth grader. Complete the explanation.

The conditional relative frequency is the number of tenth graders taking Mandarin divided by the
(select)
. The relative frequency can be represented by the expression