Last August the XT chain of gasoline stations had a temporary sales promotion in effect  In the promotion , any customer who made a purchase of ten or more gallons of gasoline was entitled to a free car wash 
For the month of August , XT experienced a ten percent increase in gasoline sales as compared to sales in August the previous year so evidently the promotion was successful as a means of boosting sales
In the areas in which XT's gasoline stations operate , how did total combined gasoline sales for all gasoline stations last August compare with sales for the previous August.