It would be 16 more 6th graders that are not in either band or orchestra. 15 6th Graders in band 9 6th Graders in both classes +18 6th Graders in orchestra ----- 42 6th Graders in either class
100 6th Graders in total -42 6th Graders in either class ------- 58 6th Graders in neither class 58-42= 16 more 6th Graders that are not in either class You're welcome :)