Final answer:
The siblings will both have the night off again on the 18th night after the given date, as the Least Common Multiple (LCM) of 9 and 6 is 18.
Step-by-step explanation:
The question asks when two siblings, one with a day off every ninth night and the other every sixth night, will both have a night off again if they both last had the same night off on a given date. This is a Least Common Multiple (LCM) problem, which can be solved by finding the smallest number that is a multiple of both 9 and 6. The multiples of 9 are 9, 18, 27, 36, and so on. The multiples of 6 are 6, 12, 18, 24, 30, 36, and so on. The first common multiple of both 9 and 6 is 18. So, the next time they will both have a night off is on the 18th night after the given date.