senyawa XO3 mengandung 40% massa unsur X. Ar X ....

%X = Ar X / Mr XO3 x 100%
40% = Ar X / ( Ar X + Mr O3) x 100%
40%/100% = a / ( a + 16 x 3 )
2/5 = a / ( a + 48 )
2a + 96 = 5a
3a = 96
a = 32
Ar X = 32