$M$ and $N$ alone can so a work in $21$ and $42$ days respectively.In how many days they can complete the work, if they work on alternate days?

14

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28

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42

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35

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Solution

The correct option is A $28$
Work completed by $M$ in one day $= \frac{1}{21}$ .

Work completed by $N$ in one day $= \frac{1}{42}$ .

So, amount of work completed in every two days $= \frac{1}{21} + \frac{1}{42} = \frac{3}{42} = \frac{1}{14}$

Let $2 d a y s = 1 t i m e u n i t .$

Number of time units required for the work to complete $\frac{1}{(\frac{1}{14})} = 14$ .

$∴$ Number of days required to complete the work $= 14 \times 2 = 28 d a y s . [B]$

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M and N alone can so a work in 21 and 42 days respectively.In how many days they can complete the work, if they work on alternate days?

$M$ and $N$ alone can so a work in $21$ and $42$ days respectively.In how many days they can complete the work, if they work on alternate days?