For the first 4 buses divide 40 by 8 to get the number of adults for those buses, it should be five.
For the bus with 4/5 of the seats filled you'll first need to find out how many seats 4/5 of the bus would amount to. To do this multiply 40 by 4/5, you should get 32. Then you divide 32 by 8 and you'll get 4.
The adults on the 5 buses would be 5 + 5 + 5 + 5 + 4 = 24.