................................ Rate .............. Time ................. Work

A ................................ \(\frac{1}{2}\) ............... 2 ....................... 1

B .............................. \(\frac{1}{3}\) .................. 3 ........................ 1

Work done by A alone in 1 hour \(= \frac{1}{2} * 1 = \frac{1}{2}\)

Pending work \(= 1 - \frac{1}{2} = \frac{1}{2}\)

Combined rate of A & B\(= \frac{1}{2} + \frac{1}{3} = \frac{5}{6}\)

Time required for A & B combine to complete the remaining work \(= \frac{\frac{1}{2}}{\frac{5}{6}} = \frac{3}{5}\)

Work done by B in the combined effort \(= \frac{1}{3} * \frac{3}{5} = \frac{1}{5}\)

Answer = A

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