1. 15 people do a piece of work in 7 days. So in how many days will 21 men do that work?
This is a very easy question, it can be solved only by addition and subtraction. It can be solved step by step in the following way.
2. What does it mean if 15 men do a work in 7 days?
This means that out of 15 men, the first man completed a certain part of the work in 7 days, and the second man also completed the same part of the work in 7 days and similarly the third and fourth all completed the same part of the work in 7 days.
3. If only one person has to do the entire work, how will he do it?
The first man will complete the work of the first man in 7 days. Then in the next 7 days he will complete the work of another person also. Then in the next 7 days he will complete the work of the third man also and in this way he will complete the work of each man in 7.7 days. In this way he will complete the entire work 7 7 7 7.....15 times in a day, which means he will complete it in 105 days.
4. If 1 man does that work in 105 days, then how will that work be done if 21 men do it.
21 men will do as much work in the first day as 1 man can do in 21 days. 21 men will do the same amount of work on the second day as 1 man did in 21 days. In this way, 21 men will do 21-21 days' work of one man every day.
5. If one man can do that work in 105 days, then in how many days will 21 men together do that work?
21 men will do as much work in the first day as 1 man can do in 21 days. So after one day 105-21 days of work will be left. So after two days, 105-21-21 days of work will be left. In this way 21 men will continue to work until the entire 105 days' work is completed. Meaning 105 -21 - 21 - 21....until the total value becomes zero. This will happen in 5 days.
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