Estimate the number of students

#1
Ques. is From the following table, estimate the number of students who obtained marks between 40 and 45
marks 30-40 40-50 50-60 60-70 70-80
No of students 31 42 51 35 31


What shall I do ? Shall I find out mean / mode or what.. what is the process. I am a Maths but never dwelt with Stats...:)