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Concave and convex mirrors

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An object is kept 10, space, start text, c, m, end text 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.
f, equals
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3, slash, 5
  • a simplified improper fraction, like 7, slash, 4
  • a mixed number, like 1, space, 3, slash, 4
  • an exact decimal, like 0, point, 75
  • a multiple of pi, like 12, space, start text, p, i, end text or 2, slash, 3, space, start text, p, i, end text
start text, c, m, end text