Let the first amount be x and the second y.

We have: `x + y = 2000` and

`x - y = 660`

Adding gives `2x = 2660` so `x = 1330`, then `y = 670`.

Checking: `x + y = 1330 + 670 = 2000` (OK)

`x - y = 1330 - 670 = 660` (OK)

So the amounts of dosage should be `1330` mg for the adult and `670` mg for the child.