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If ρ is the density and η is coefficient of viscosity of fluid which flows with a speed v in the pipe of diameter d, the correct formula for Reynolds number Re is:

1. Re = ηd / ρv
2. Re = ρv / ηd
3. Re = ρvd / η
4. Re = η / ρvd

Correct answer: Re = ρvd / η

Solution

The Reynolds number is a dimensionless quantity that characterizes the flow of fluid, and it is defined as the ratio of inertial forces to viscous forces. The formula Re = ρvd / η correctly incorporates the fluid density (ρ), velocity (v), pipe diameter (d), and viscosity (η) to represent this relationship.

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