For b you had to add 25 and 35 together to make 60.
Divide 60 by 2 to get 30. 30 is the average between the two numbers. You then find the frequency of 30 using the graph. For the 12 year olds it is 40 and for the 18 year olds it is 60. Find the difference between these 2. So 60-40= 20