Answer
$2\times10^1\ mi/gal$
Work Step by Step
$\frac{400\ km}{47.3\ L}\times\frac{1\ mi}{1.6093\ km}\times\frac{1\ L}{1.057\ qt}\times\frac{4\ qt}{1\ gal}=\frac{19.9\ mi}{gal}\approx2\times10^1\ mi/gal$
estimate
$400\div50=8$
$8\div1.6=5$
$5\div1=5$
$5\times4=20$
