Task: Question Answering
Input (English): Problem: a is twice efficient as b and together they do the same work in as much time as c and d together . if c and d can complete the work in 20 and 30 daysrespectively , working alone , then in how many days a can complete the work individually : 
Options: a ) 12 days , b ) 18 days , c ) 24 days , d ) 30 days , e ) none of these
Output (English): b