The can contains 15.7 g of sugar per 100 mL. So if the can holds 330 mL of soft drink, then overall it contains
(15.7 g) / (100 mL) × (330 mL) = 51.81 g
of sugar, and this is
(51.81 g) / (90 g) ≈ 0.575667 ≈ 57.6%
so Tom's friend is correct.
