PUZZLE 019 School Sister

Two sisters attend the same school.
Each morning, the younger sister sets off first, and the older sister leaves the house when the younger sister has travelled one hundred maters.
The younger sister walks at one metre per second, and the older sister walks at two meters per second.
What is the minimum number of seconds it would take the older sister to catch up?
They live in the same house travel the same route, and neither of them turn back on their way to school.

Hint 1

If they both keep walking, the older sister will get one metre closer to the younger sister every second, so she would catch up with her after one hundred seconds.
However, there is a way she could catch up sooner.

Hint 2

One detail we don't know here is how far away the school is.
For the older sister to catch up in the shortest possible time, where would the school have to be?

Hint 3

If the school is located as close as possible, it will take less time for the older sister to catch up.

Super Hint

If the school was located one hundred metres from the house, the younger sister would already have arrived at her destination when the older sister set off.

Answer for 'School Sister'

The answer is '50'.