Please help and first right answer will be marked branliest

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