A lethal dose would be 300 cans of soda.
This is a problem in unit conversion.
You have to convert grams of caffeine → milligrams of caffeine → ounces of soda → cans of soda.
10.0 g caffeine ×
300 cans soda
Notice how the conversion factors are set up to make the units cancel and give the desired units for the answer.
Note: The answer can have only 2 significant figures, because that is all you gave for the ounces of soda in a can. If you need more precision, you will have to recalculate.