Two pipes A and B can fill a tank in 10 hours and 12 hours respectively. A third pipe C can empty the tank in 20 hours. If all three pipes are opened and function simultaneously, how much time (in hours) will the tank take to be full?
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Using the "Total Capacity" (LCM of time periods) method is often faster and less error-prone than adding fractions.
Setting the capacity as 60 units directly turns fractional additions into simple integer arithmetic ($6 + 5 - 3 = 8$).
Then simply divide $60 / 8 = 7.5$.