$[15]$ A vapor contains $20\%$ hexane, $75\%$ heptane and $5\%$ nonane by volume on an air free basis. The

lower flammable limits are $1\mathrm{.}2,1\mathrm{.}4$ and $0\mathrm{.}7$ volume percent fuel in air respectively. What is the lower

flammable limit of this mixture in $[$vol$.\%$ fuel in air$]?$