A decilitre of water is approximately 20 teaspoons

The equivalent of 15 decilitre is 1 500 millilitres.

You can't associate volume and length. I'll make an assumption that you either meant millimetres cubed or millilitres. There are 100,000 millimetres cubed in a decilitre. There are 100 millilitres in a decilitre.

There are 10 decilitres in one litre. Therefore, 1 decilitre is equal to 1/10 = 0.1 litres.

There are 100 millilitres in one decilitre. Therefore, 100 millilitres is equal to 100/100 = 1 decilitre.