Pipe A can fill a tank in 36 minutes and pipe B can fill it in 45 minutes. If both pipes are opened at the same time, after what time should pipe A be closed so that the whole tank is filled in 25 minutes?

Pipe A fills at 1/36 per minute, Pipe B at 1/45 per minute. Combined rate = 1/36 + 1/45 = 5/180 + 4/180 = 9/180 = 1/20 per minute. Let A be open for t minutes. Work done together = t/20, remaining work by B = (25-t)/45. Tank filled when: t/20 + (25-t)/45 = 1. Solving: 9t + 4(25-t) = 180, 5t = 100, t = 16. Verification: 16/20 + 9/45 = 4/5 + 1/5 = 1 ✓