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Two pipes A and B can fill a tank in 6 hrs and 4 Hrs respectively. If they are opened on alternate hrs and Pipe A is opened first, in how many hours shall the tank be full ..?

  • A.4 Hrs
  • B.4 1/2 Hrs
  • C.5 Hrs
  • D.5 1/2 Hrs

Answer: C

Run the hours in pairs, since A and B alternate with A first. Each pair of hours fills

\[ \frac{1}{6} + \frac{1}{4} = \frac{2 + 3}{12} = \frac{5}{12} \]

After two full pairs, that is 4 hours:

\[ 2 \times \frac{5}{12} = \frac{5}{6} \]

leaving \( \tfrac{1}{6} \). The fifth hour belongs to A, which fills exactly \( \tfrac{1}{6} \) in an hour, so the tank is full at the end of it:

\[ \text{total} = 5 \text{ hours} \]

The remainder happening to match A's hourly output is what makes the answer a whole number here.

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