Observe the ruler, it is marked in centimeters.

The long marks are centimeters, and the shorter marks are millimeters and

there are 10 millimeters (mm) for each centimeter (cm).

The keyt starts at 0^{th} short mark and ends at 33^{th} short mark.

Length of key = 33 millimetres.

= 30 mm + 3 mm (Rewrite 33 as 30 + 3)

= 3cm + 0.3 cm (10 millimetres = 1 centimetre)

= 3.3 cm

The length of the key is 3.3 cm.

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