Task: Question Answering
Input (English): Problem: if a farmer sells 15 of his goats , his stock of feed will last for 4 more days than planned , but if he buys 20 more goats , he will run out of feed 3 days earlier than planned . if no goats are sold or bought , the farmer will be exactly on schedule . how many goats does the farmer have ? 
Options: a ) 12 , b ) 24 , c ) 48 , d ) 60 , e ) 55
Output (English): d