Assume total number of students = 100
we know that 40 percent are 3rd yr students so 100*.4 = 40 3rd yr students.
this means there are 60 non-3rd yrs. (could be 1st, 2nd, or 4th)
so (# 1st yrs) + (# 2nd yrs) + (# 4th yrs) = 60
we also know that 70 are not 2nd yrs.
so (# 1yrs) + (#3rd yrs) + (# 4th yrs) = 70
but we know from the 1st statement that # 3rd yrs = 40
then the 2nd equation becomes (#1 yrs) + 40 + (# 4th yrs) = 70
---> 1st yr + 4th yrs = 30
plug this back to 1st equation
--> 30 + 2nd yrs = 60
--> 2nd yrs = 30
we want the fraction of (# 2nd yrs) / (# non 3yrd yrs ) = 30/60 = 1/2