The Vernier scale of a travelling microscope has 50 divisions which coincides with 49 main scale divisions. If each main scale division is $$0.5 \mathrm{~mm}$$, then the least count of the microscope is

A cylindrical wire has a mass $$(0.3 \pm 0.003) \mathrm{g}$$, radius $$(0.5 \pm 0.005) \mathrm{mm}$$ and length $$(6 \pm 0.06) \mathrm{cm}$$. The maximum percentage error in the measurement of its density is

If $$P, Q$$ and $$R$$ are physical quantities having different dimensions, which of the following combinations can never be a meaningful quantity?