Task: Question Answering
Input (English): Problem: four 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 ) 5 , b ) 3 , c ) 4 , d ) 6 , e ) 1
Output (English): c