Time and Work: Pipe A fills a tank in 10 hours, Pipe B in 15 hours. Both are opened for 4 hours, then Pipe A is closed. How much time will Pipe B take to fill the remaining part?

Pipe A’s rate = 1/10 tank/hour Pipe B’s rate = 1/15 tank/hour

In 4 hours together: → Work done = 4 × (1/10 + 1/15) = 4 × (5/30) = 2/3 of the tank

Remaining = 1 − 2/3 = 1/3 of the tank

Pipe B fills 1/15 per hour → Time to fill 1/3 = (1/3) ÷ (1/15) = 5 hours

Answer: Pipe B takes 5 hours to fill the remaining part.