Task: Question Answering
Input (English): Problem: fox jeans regularly sell for $ 15 a pair and pony jeans regularly sell for $ 18 a pair . during a sale these regular unit prices are discounted at different rates so that a total of $ 4 is saved by purchasing 5 pairs of jeans : 3 pairs of fox jeans and 2 pairs of pony jeans . if the sum of the two discounts rates is 18 percent , what is the discount rate on pony jeans ? 
Options: a ) 45 % , b ) 13 % , c ) 11 % , d ) 12 % , e ) 15 %
Output (English): a