### Brain Teasers

# 13 cm Ruler

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 can read off all full centimeter 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.

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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