8 classes x 20 students per class = 160 students
8 classes x 2 chaperones per class = 16 chaperones
Total people on the buses . . . . . . . . . . 176 people
The expression for the number of buses needed is:
Number of buses = 1 + (the greatest integer in 176/25)
That's because if there are any people left over after the
last full bus of 25, they will need one more bus.
Unfortunately for the school's budget, this group needs
7 full buses carrying 25 people in each, and one bus
carrying one single person in it.
A possible solution to the problem would be to find somebody
who is certified as a bus driver and willing to work two jobs
on the field trip, to act as one of the chaperones. Then they
might get away with only 7 buses.
If this doesn't work, then they can just forget about the whole
idea of 25 on a bus. Loading them to capacity doesn't help,
because it still leaves one person without a ride. They might
as well just put one class and their chaperones on each of the
8 buses, and give each load a little bit of room to spread out.
If there's one student out of 160 who is sick or otherwise doesn't
show up on field-trip day, then they can squeeze everybody into
7 fully-loaded buses. But since they had to reserve 8 buses and
drivers, they'll probably have to pay for the eighth one anyway.