How many times does a 250ml glass need to be used fill 20L aquarium?

Here's how to solve this:

1. Convert units:

* We need both volumes to be in the same units. Let's convert liters (L) to milliliters (ml):

* 20 L * 1000 ml/L = 20,000 ml

2. Divide the total volume by the glass volume:

* 20,000 ml / 250 ml/glass = 80 glasses

Answer: You would need to use the 250 ml glass 80 times to fill a 20 L aquarium.