Task: Question Answering
Input (English): Problem: two pipes can fill a tank in 12 minutes and 20 minutes respectively . both pipes are opened together and after some time the first pipe is closed and the tank is full in totally 10 minutes . for how many minutes was first pipe open ? 
Options: a ) 8 minutes , b ) 6 minutes , c ) 7 minutes , d ) 10 minutes , e ) 9 minutes
Output (English): b