Pipe A can fill a tank in 12 hrs, pipe B can fill in 15 hrs where as pipe C can fill in 20 hrs. Ramesh woke up at 6 AM and turned pipe B on ,he again came at 9:30 AM and turned A on and finally at 11:30 AM turned C also on. At what time tank will be completely filled?

Pipe rates: A = 1/12 = 8.33%/hr, B = 1/15 = 6.67%/hr, C = 1/20 = 5%/hr. From 6:00-9:30 (3.5hr): B fills 3.5 × 6.67% = 23.33%. From 9:30-11:30 (2hr): A+B fills 2 × (8.33% + 6.67%) = 30%. After 11:30: All three fill at 20%/hr. Remaining = 100% - 23.33% - 30% = 46.67%. Time = 46.67% / 20% = 2.33hr = 2hr 20min. Total = 6:00 + 3hr 30min + 2hr + 2hr 20min = 1:50 PM.