Answer:
a) 320 students
b) 25 students
Step-by-step explanation:
Number of 18-years old in the survey:
(We add all frequency points for each line)
0 + 30 + 20 + 60 + 70 + 40 + 60 + 40 + 0 = 320 students
Number of 18 year old spending between 25 and 35 minutes: 40 + 65 = 105
Number of 12 year old spending between 25 and 35 minutes: 30 + 50 = 80
105 - 80 = 25 students
