So in this problem the cost of the jeans is being represented by the number of dollar icons. Jean 'a' is worth 10 dollars, while Jean 'b' is worth 18 dollars. Since it is asking what percent of Jean 'a' is that cost of Jean 'b', you want do divide the cost of b by the cost of a.
This leaves us with a ratio that represents what multiple of a b is. At this point the work is mostly done, in these questions when you have the ratio, the last step is to multiply by 100 to convert the ratio to a percentage. This leaves us with: