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Bohr orbits: orbital radius and orbital speed

Problem

According to Bohr's model of the hydrogen atom, the radius of the fourth orbital, r4=8.464 .
(Imagine how tiny that is compared to the shortest wavelength of visible light, which is around 3800 .)
What is the radius of the fifth orbital, r5?
Note: Let's use only the given value of r4, and input the answer upto three decimal places.
r5=
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi