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Two rates combined: together time from individual times or one time from the others

This type of question, asked 5 times: CDS 1 2017, CDS 2 2018, CDS 1 2019, CDS 2 2019, CDS 1 2020. All 5 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    CDS 1 2017

    28
    A and B working together can finish a piece of work in 12 days while B alone can finish it in 30 days. In how many days can A alone finish the work ?

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  2. Question 2

    CDS 2 2018

    44
    X and Y together can finish a job in 6 days. X can alone do the same job in 12 days. How long will Y alone take to do the same job ?

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  3. Question 3

    CDS 1 2019

    79
    Two taps XX and YY are fixed to a water tank. If only XX is opened, it drains out the full tank of water in 20 minutes. If both XX and YY are opened, then they drain out the full tank of water in 15 minutes. If only YY is opened, how long does it take to drain out the full tank of water ?

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  4. Question 4

    CDS 2 2019

    40
    Tushar takes 6 hours to complete a piece of work, while Amar completes the same work in 10 hours. If both of them work together, then what is the time required to complete the work ?

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  5. Question 5

    CDS 1 2020

    48
    In a water tank there are two outlets. It takes 20 minutes to empty the tank if both the outlets are opened. If the first outlet is opened, the tank is emptied in 30 minutes. What is the time taken to empty the tank by second outlet?

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