With a blank ruler (without marks) of exactly 13 cm you can only measure 13 cm. Now you are allowed to set 4 marks so that you may read off all full centimetre values from 1 to 13 cm directly from the ruler.
Where must these 4 marks be?
Much harder question:
The blank ruler has a length of 36 cm and you need 8 marks.
Set the marks.
Answer
Marks on the 13-cm ruler:
1 – 2 – 6 and 10 cm.
You could then use these marks together to measure any length of 13 cm or less. For example, you could measure 8 cm by using the distance between the 2 and 10 cm marks.
Marks on the ruler with 36 cm:
1 – 3 – 6 – 13 – 20 – 27 – 31 – 35 cm
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