Answer: 90 Earth years.
Step-by-step explanation:
Analizing the information provided in the exercise, you need to find the Least Common Multiple (LCM) of the given numbers.
You can follow these steps:
1. You must descompose 6, 9, 15 and 18 into their prime factors:
2. Finally, you need to choose the commons and non commons with their greatest exponents and multiply them. Then you get:
Therefore, it will take 90 Earth years for them to all line up.