480700. The different combinations of students that could go on the trip with a total of 25 student, but only 18 may go, is 480700.
The key to solve this problem is using the combination formula 
. This mean the number of ways to choose a sample of r elements from a set of n distinct objects where order does not matter and replacements are not allowed.
The total of students is n and the only that 18 students may go is r:
2(4z-9-11)=166-46
8z-18-22=120
8z-40=120
Add 40 from both side
8z-40+40=120+40
8z=160
Divided 8 from both side
8z/8=160/8
z=20. Hope it help!
So what u do is (29272829)+(-8382929)=(-7398)-n
