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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 R e is:

1. R e = v d ρ η
2. R e = d v η ρ
3. R e = η ρ vd
4. R e = vd ρ η

Correct answer: R e = η ρ vd

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