Task: Question Answering
Input (English): Problem: working alone , a can complete a certain kind of job in 15 hours . a and d , working together at their respective rates , can complete one of these jobs in 10 hours . in how many hours can d , working alone , complete one of these jobs ? 
Options: a ) 20 , b ) 24 , c ) 26 , d ) 28 , e ) 30
Output (English): e