An element X is found to combine with oxygen to form X₄O₆. If 8.4 g of this element combine with 6.4 g of oxygen, the atomic weight of the element in u is

$4\mathrm{X}+3{\mathrm{O}}_2\to {\mathrm{X}}_4{\mathrm{O}}_6$

6.4 g of oxygen and 8.4 g of X is required to form X₄O₆. 

Mass of 3 moles of O₂ is $96 \left(3\times 32\right)$ and the mass of X requires to react with 96 g of O₂ is:

$\mathrm{Mass} \mathrm{of} \mathrm{X}=\frac{8.4}{6.4}\times 96=126 \mathrm{g}$

Mass of 4 moles of X is 84 g and the mass of one mole of X is 31.5 g.