Observe the ruler, it is marked in centimeters.

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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.