The price of the soda in this problem is elastic. It was stated in the problem that if you drop the price and the revenue goes up then it the price is elastic. You have to compare the revenue of the $2 soda with the revenue of the $1 soda.
$2 x 10 sodas = $20
$1 x 30 sodas = $30
In here, the revenue obviously went up even if the price was drop. Therefore, the price is elastic.