A sunscreen preparation contains 2.40 % by mass benzyl salicylate. If a tube contains 3.0 oz of sunscreen, how many kilograms of benzyl salicylate are needed to manufacture 425 tubes of sunscreen?

You need to know that:

1 ounce = 28.35 grams

So, 1 tube contains

3.0 oz * 28.35 = 85.05g total mass

Mass of Benzyl salicylate =

2.40/100 = Percentage of Benzyl salicylate (in numerical value)

= 0.024

0.024 * 84 = Mass of Benzyl Salicylate

=2.016g

If one tube contains

2.016g * 425 tubes

=856.8g

Then,

Converting into kg by (dividing by 1000)

=856.8 : 1000

= 0.8568 grams

= 0.857g (3sf)