620 miles / 20 gallons = 31 miles/gallon
($1.43 / gallon) / (31 miles / gallon) = ($1.43 / gallon) * (1 gallon / 31 miles)
($1.43 / gallon) * (1 gallon / 31 miles) = $1.43 / 31 miles
$0.046 / mile <<<<< This is rounded down, so the next operation won't be exact
620 miles * ($0.046 / mile) = $28.52
($1.43 / gal) * 20 gal = $28.60