X and Y are two volatile liquids with molar weights of $10\; g\; mol^{–1}$ and $40\; g\; mol^{–1}$ respectively. Two cotton plugs, one soaked in X and the other soaked in Y, are simultaneously placed at the ends of a tube of length $L = 24\; cm$, as shown in the figure. The tube is filled with an inert gas at 1 atmosphere pressure and a temperature of $300\; K$. Vapours of X and Y react to form a product which is first observed at a distance d cm from the plug soaked in X. Take X and Y to have equal molecular diameters and assume ideal behaviour for the inert gas and the two vapours.The value of d in cm (shown in the figure), as estimated from Graham’s law, is :

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