Explain the difference between a base unit and a derived unit

A SI base unit is the single and direct measurement unit for a physical entity. E.g. For mass it is kg, for time it is sec, A derived unit is determined by a physical equation with the base units, like for velocity equals distance divided by time, in SI units: unit of v (velocity)= m (meter) divided by sec (seconds).

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The difference of base unit in derived unit is any fundamental units of length , mass, time etc. while the derived unit is density is the kilo gram per cubic metre
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