Two pipes C and D can fill a tank in 6 hours and 9 hours, respectively. They are opened alternately for | hour each, starting with C first. In what duration, will the tank be filled? 

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  1. 8 hours
  2. 5 hours 
  3. 6 hours 
  4. 7 hours 

Answer (Detailed Solution Below)

Option 4 : 7 hours 
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Detailed Solution

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Given:

Pipe C fills tank in 6 hours.

Pipe D fills tank in 9 hours.

Pipes are opened alternately for 1 hour each, starting with C.

Formula used:

LCM method to find total capacity.

Efficiency = Total capacity / Time.

Calculations:

LCM of 6 and 9 = 18 (Total capacity).

Efficiency of C = 18 / 6 = 3 units/hour.

Efficiency of D = 18 / 9 = 2 units/hour.

In 2 hours (C then D), work done = 3 + 2 = 5 units.

In 6 hours (3 cycles of 2 hours), work done = 5 × 3 = 15 units.

Remaining work = 18 - 15 = 3 units.

Next hour, C works and fills 3 units.

Total time = 6 + 1 = 7 hours.

∴ The tank will be filled in 7 hours.

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