A coaching institute has students in 3 batches – X, Y and Z. In a certain examination, the average marks obtained by these batches are 72, 60 and 50 respectively. The average marks of batches X and Y taken together is 69. If the ratio of the number of students in batches Y and Z is 6 : 7, what is the average score of all the three batches put together? (a) 59       (b) 61.6      (c) 63.5     (d) 64.7      (e) 65