Pipe A takes 36 hours to fill a tank. Or Pipe A fills (1/36)th of the tank in 1 hour.Pipe B takes 46 hours to fill a tank. Or Pipe B fills (1/46)th of the tank in 1 hour.So Pipes A and B together fill (1/36)+(1/46) = (46+36)/(36*46) or (1/20.19512195)th part of the tank in 1 hour.Hence Pipes A and B together will fill the entire tank in 20.19512195 hours.