Workers take turns (alternate days or hours): time to finish

This type of question, asked 2 times: CDS 1 2024, CDS 1 2026. All 2 questions below, oldest first.

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Question 1
CDS 1 2024
20 $A, B, C, D can complete a work in 3, 6, 9, 12 hours respectively. Further, only one person can work at a time in each hour and nobody can work for two consecutive hours. It is not necessary to engage all. What is the minimum number of hours that they will take to finish the work ?$
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Question 2
CDS 1 2026
14 $X can do a piece of work in 4 days and Y can do the same piece of work in 6 days. Which of the following statements is/are correct ? I. If X and Y work alternately starting with X, then the piece of work will be finished on the fifth day. II. If X and Y work alternately starting with Y, then the piece of work will be finished in less than 5 days. Select the answer using the code given below :$
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Question 1