Explanation:
x = number of weeks
so, we need to see, what is the number of weeks it takes for both to have the same balance.
it takes both the same amount of weeks, of course.
so,
250 + 15x = 750 + 10x
15x = 500 + 10x
5x = 500
x = 100
it takes them 100 weeks to have the same amount of money in their accounts.