Standard Deviation Question....Help?

Incoming students take a mathematics exam consisting of 85 multiple choice questions. Suppose the following is the distribution of the scores on the exam: Test Score 5-25 25-45 45-65 65-85 Number of Students 20 36 38 6 Find the sample mean score: a) 41 b) 42 c) 38 d) 37 e) 37.5 The answer is 41 but I cannot figure out how this answer was derived....can somebody please help? Find the standard deviation: a) 17.08 b) 17.17 c) 15 d) 292 e) 294.9 Any help would be very much appreciated...please show your work because I have another problem just like this that I need to do on my own...thank you!!!