Question

What is the mass of aluminium in 306 g of aluminium oxide Al2O3?​

Answer 1

in 102 g Al2O3 is 54g Al and 48g O

in 3×102g Al2O3 is 3×54g Al and 3×48g O

i.e.in 306g aluminium oxide there is 162g Al and 144 g O.