Suppose we write "week" as a variable, because that's what we want to find. Let's say w.
Then, if you earn 100 dollars per week, the amount of money earned after w weeks would be 100w. We want to find how long it takes to earn 100,000 dollars. We would write:
100w = 100,000
Divide 100 from both sides:
w = 1000 weeks
The answer is 1000 weeks.