Task: Question Answering
Input (English): Problem: a furniture manufacturer has two machines , but only one can be used at a time . machine w is utilized during the first shift and machine b during the second shift , while both work half of the third shift . if machine w can do the job in 12 days working two shifts and machine b can do the job in 15 days working two shifts , how many days will it take to do the job with the current work schedule ? 
Options: a ) 14 , b ) 13 , c ) 11 , d ) 9 , e ) 7
Output (English): d