How the linear velocity v of an electron in the Bohr orbit is related to its quantum number n?

If the half-life of a radioactive nucleus is 3 days, nearly what fraction of the initial number of nuclei will decay on the third day? (Given, $$\root 3 \of {0.25} \approx 0.63$$)

An electron accelerated through a potential of 10000 V from rest has a de-Broglie wave length $$\lambda$$. What should be the accelerating potential, so that the wavelength is doubled?

The de-Broglie wavelength of an electron is $$0.4 \times {10^{ - 10}}$$ m when its kinetic energy is 1.0 keV. Its wavelength will be $$1.0 \times {10^{ - 10}}$$ m, when its kinetic energy is