The cost of renting a room has two terms:
1) A fixed cost of $18.
2) An additional fee of $6 per hour.
Then, if t is the number of hours, we can write the cost as:
Then, if the maximum cost is $90 for this group, we can find the maximum number of hours as:
The number of hours has to be equal to or less than 12.
Answer: t ≤ 12