Men x days x hours per unit of work is constant: find the missing quantity

This type of question, asked 6 times: CDS 1 2017, CDS 1 2018, CDS 2 2018, CDS 2 2020, CDS 1 2021, CDS 1 2026. All 7 questions below, oldest first.

Try each one first, then open its answer.

Question 1
CDS 1 2017
24 $If 15 men take 21 days of 8 hours each to do a piece of work, then what is the number of days of 6 hours each that 21 women would take, if 3 women would do as much work as 2 men ?$
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Question 2
CDS 1 2018
32 $If 5 tractors can plough 5 hectares of land in 5 days, then what is the number of tractors required to plough 100 hectares in 50 days?$
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Question 3
CDS 2 2018
45 $Twelve (12) persons can paint 10 identical rooms in 16 days. In how many days can 8 persons paint 20 such rooms ?$
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Question 4
CDS 2 2018
40 $Twelve (12) men work 8 hours per day and require 10 days to build a wall. If 8 men are available, how many hours per day must they work to finish the work in 8 days ?$
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Question 5
CDS 2 2020
18 $x$
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Question 6
CDS 1 2021
39 $If 20 persons can clean 20 floors in 20 days, then in how many days can 16 persons clean 16 floors?$
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Question 7
CDS 1 2026
15 $12 men working 8 hours a day can completely build a wall of length 100 m, breadth 20 cm and height 5 m in 10 days. How many days will 16 men working 10 hours a day require to build a wall of length 200 m, breadth 60 cm and height 5 m ?$
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Question 1