Answer:
There is 40*10⁻¹² g (or 4 * 10⁻¹¹ g) solute in one liter of solution
Step-by-step explanation:
Concentration measured in very small levels is expressed in terms of ppt i.e. parts per trillion which is the amount (grams) of solute present in a liter (L) volume of solution

The concentration of salt in the aquarium is 40 ppt
This implies that there is 40*10⁻¹² g salt in one liter of the solution