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Astronomical Telescope

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A detective uses a telescope to spy on one of his suspects.
He focuses the image in such a way that his eyes are relaxed, so he knows that the final image is formed at infinity. He maintains the length of his telescope tube at 65 cm. The focal length of the objective and eyepiece lenses are also known to him, fo=50 cm and fe=10 cm.
Find the distance of the suspect from his telescope's objective lens.
  • 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