Ansen and Beirul drank all the water in a bottle of water. Ansen drank 100 ml more than $\frac{3}{8}$ of the total amount of water in the bottle. Beirul drank 250 ml. How much water was there in the bottle of water at first?

1. Determine the fraction of water that corresponds to the remaining amount after taking away 3/8 of the total.

Total water $= 1 = \frac{8}{8}$
Remaining fraction $= 1 - \frac{3}{8} = \frac{5}{8}$

2. Equate the remaining fraction to the actual volume of water left.

The remaining $\frac{5}{8}$ of the water is made up of the extra $100$ ml Ansen drank and the $250$ ml Beirul drank.
$\frac{5}{8}$ of total $= 100 + 250 = 350$ ml

3. Find the total amount of water in the bottle.

$5$ units $= 350$ ml
$1$ unit $= 350 \div 5 = 70$ ml
Total ($8$ units) $= 70 \times 8 = 560$ ml