Task: Question Answering
Input (English): Problem: two fill taps a and b can separately fill a cistern in 45 and 40 minutes respectively . they started to fill a cistern together but tap a is turned off after few minutes and tap b fills the rest part of cistern in 23 minutes . after how many minutes , was tap a turned - off ? 
Options: a ) 9 min , b ) 10 min , c ) 12 min , d ) 7 min , e ) none of these
Output (English): a