Final answer:
To find the probability, divide the number of 6th graders who prefer fishing by the total number of students.
Step-by-step explanation:
In order to find the probability of a student being a 6th grader given that their favorite leisure activity is fishing, we need to know the total number of students and the number of students in each grade who prefer fishing.
Let's assume that there are 100 students in total and the number of 6th graders who prefer fishing is 20.
The probability of a student being a 6th grader given that fishing is their favorite leisure activity is calculated by dividing the number of 6th graders who prefer fishing by the total number of students: 20/100 = 0.2 or 20%.