Answer: 27 logs
Explanation:
Given : Teresa is maintaining a camp fire. She can keep the fire burning for 4 hours with 6 logs.
From the given situation , it is clear that the number of log and time for fire burning are directly proportional.
The equation for direct variation :-

Put ,
, where y is the number of logs need to keep the fire burning for 18 hours.

Hence,she needs 27 logs to keep the fire burning for 18 hours.