$6.$ A reactor in a pesticide plant contains $1000$lb of a liquid mixture of $50\%$ by weight liquid methyl isocyanate $($MIC$).$ The liquid is near its boiling point. A study of various release scenarios indicates that a rupture of the reactor will spill the liquid into a boiling pool on the ground. The boiling rate of MIC has been estimated to be $20$ lb$/$min$.$ Evacuation of the population must occur in areas where the vapor concentration exceeds ERPG$-3.$

a$.$ If the wind speed is $3\mathrm{.}4$ mph on a clear night, estimate the area downwind that must be evacuated.