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.
Let y and z be the no. of students that walk to school, that takes the subway and that takes the bus
We have,
3/y = 1/2
=> y = 6 --- (1)
and,
y/z = 1/3
=> y/z = 1/3 [from (1)]
=> z = 18
Hence the no. of students that takes the bus are 18 students
If the bases are the same just add the exponents
Answer:
$16.20 30% of 50 is 15 so divide 4 in the mix