$\ce{FeO}$ = 70 wt%
$\ce{TiO2}$ = 30 wt%
The molecular weight given of $\ce{FeO}$ = 72, and the mw of $\ce{TiO2}$ = 80.
The first step I did was divide each wt% by their molecular weights to get a molecular proportion:
$$\ce{FeO}: {70 \over 72} = 0.972 $$
$$\ce{TiO_2}: {30 \over 80} = 0.375$$
Then, I multiplied each proportion by the number of oxygens in each chemical formula
$$0.972 \times 1 \ \rm{oxygen} = 0.972$$ $$0.375 \times 2 \ \rm{oxygens} = 0.75$$
The next step is where I get really confused. I think I'm supposed to add the two numbers above together, and divide that number into a number that is supposedly the cations assumed of the mineral? (As in 5 for plagioclase, 4 for pyroxene, 3 for olivine, etc) But I do not understand how I'm supposed to gather which number to use based on only the data given. If anyone can point me in the right direction it would be greatly appreciated.