There are 1000 metres in a kilometre and 100 centimetres in a metre... so there are 100,000 centimetres in a kilometre.
This means that a speed of ? Km per hour = 100,000 x ? cm per hour.
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To answer a question like this, I usually draw a line (representing a fraction bar). everything on top gets multiplied to form a numerator; everything on bottom is multiplied to get a denominator,...