Question

Every weekend, Trudy bikes to the park to play disc golf with her friends. Before leaving home, she fills her water bottle to the top. This weekend, Trudy drinks
3/8 of a liter on the way to the park. Now there is only
1/8 of a liter remaining in Trudy's water bottle.

Answer 1

This is the kind of problem we can "work backwards" to find the answer.
So, we know at the end, there's 1/8 liter of water left. And we know that Trudy drank 3/8 liter from when the bottle was full at the beginning.
So, if we want to get from the end back to the beginning, we'll "work backwards" by adding back the water that Trudy drank.
1/8 liter (remaining) + 3/8 liter (that Trudy drank) = how much water at the beginning?

Actually, I just assumed that's what the question was asking. It doesn't seem like there is a direct question in your post. Did I assume correctly?

Answer 2

that just hurt meh brain