I believe this is to find the median.
To find the median you would line these gasoline prices up from smallest to biggest. Seeing as there are 7 prices there will only be one median so no need to, but if there was two middle numbers then you would have to add them both up and find the middle of both of them.
To find the median in this case:
$1.61, $1.75, $1.79, $1.84, $1.96, $2.09, $2.11
Median is $1.84
To find the difference from the 1st ($1.61) and 3rd ($1.79) quartiles
$1.79 - $1.61 = $0.18