Task: Question Answering
Input (English): Problem: pipe a fills a tank of capacity 850 liters at the rate of 40 liters a minute . another pipe b fills the same tank at the rate of 30 liters a minute . a pipe at the bottom of the tank drains the tank at the rate of 20 liters a minute . if pipe a is kept open for a minute and then closed and pipe b is open for a minute and then closed and then pipe c is open for a minute and then closed and the cycle is repeated , when will the tank be full ? 
Options: a ) 42 minutes , b ) 51 minutes , c ) 39 minutes , d ) 40 minutes 20 seconds , e ) none of these
Output (English): b