Since the fish holds 350 liters of water, it follows that,
350/5=70 liters X 2 drops for every 5 liters=140 drops needed for the whole tank.
Since 1 cupful contains 40 drops, the we have this:
140 drops needed for the full tank/40 drops contained in one cup, or,
140/40=3 1/2=3.5 number of cupfuls needed for the full tank.