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Denote the Avogradro's constant as L.
Let x be the relative atomic mass of X.
In 1 mole of XO3, no. of atoms = 1 * 4 * L = 4L
In 72g of XH4, no. of atoms = 72/(x + 4) * 5 * L = 360L/(x + 4)
Since no. of atoms in XO3 = no. of atoms in XH4
4L = 360L/(x + 4)
4x + 16 = 360
x = 86
Thus the relative atomic mass of X is 86.