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Concave and convex mirrors

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An object is kept 10 cm in front of a convex mirror, which forms a smaller image three-fifths the size of the object.
What is the focal length f of the mirror?
Note: Convex mirrors form only virtual and erect images.
Let's write the answer with the correct sign.
  • 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