Task: Question Answering
Input (English): Problem: five machines , each working at the same constant rate , together can complete a certain job in 20 days . how many additional machines , each working at the same constant rate , will be needed to complete the job in 10 days ? 
Options: a ) 4 , b ) 6 , c ) 8 , d ) 5 , e ) 10
Output (English): d