A model aircraft with a rectangular wing is tested in sea level conditions under a wind tunnel. The velocity of the flow is $300$ m$/$s and the wing span and chord is $30$ ft and $1\mathrm{.}5$ m respectively. The viscosity for sea level is given by $1\mathrm{.}7894$ x $10-5,$ if ms the aircraft is tested in actual scale with density corresponding to $7000$ ft$,$ velocity of $600$ m$/$s$,$ and the same Reynold's number, what should be the wing chord of the aircraft?