Seven twelves of 840 students went to a spring concert. how many did not go

There are several information's of immense importance already given in the question. based on those given information's the answer to the question can be easily derived.
Total number of students = 840
Then
Number of students going to the concert = (7/12) * 840
                                                                  = 7 * 70
                                                                  = 490 So
Number of students not going to the spring concert = 840 - 490
                                                                                   = 350
So 350 students did not go to the spring concert.I hope the procedure is clear enough for you to understand. You can always use this procedure for solving similar problems in future without requiring any help from outside.