Average speed as total distance over total time for unequal legs

This type of question, asked 4 times: CDS 2 2017, CDS 2 2018, CDS 2 2019, CDS 1 2021. All 4 questions below, oldest first.

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Question 1
CDS 2 2017
93 $A man travelled 12 km at a speed of 4 km/hr and further 10 km at a speed of 5 km/hr. What was his average speed ?$
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Question 2
CDS 2 2018
29 $A cyclist covers his first 20 km at an average speed of 40 kmph, another 10 km at an average speed of 10 kmph and the last 30 km at an average speed of 40 kmph. Then the average speed of the entire journey is$
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Question 3
CDS 2 2019
31 $A race has three parts. The speed and time required to complete the individual parts for a runner is displayed on the following chart : Part I Part II Part III Speed (kmph) 98 7.5 Time (minutes) 5080100 What is the average speed of this runner ?$
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Question 4
CDS 1 2021
34 $A train travels 600 km in 5 hours and the next 900 km in 10 hours. What is the average speed of the train?$
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	Part I	Part II	Part III
Speed (kmph)	9	8	7.5
Time (minutes)	50	80	100

Question 1