Pipe T1 can fill a cistern alone in 20 hours while another Pipe T2

Pipe T1 can fill a cistern alone in 20 hours while another Pipe T2 alone can empty the tank in 24 hours. If Pipe T2 is opened after 15 hours of starting of Pipe T1, then in how much time the tank will be filled? (a) 40 hours (b) 45 hours (c) 44 hours (d) 50 hours (e) 20 (…)