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Image formed by multiple lenses

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Between an object and a screen are two lenses as shown below.
The object is located 18 cm from the first lens of focal length f1=+15 cm. The second lens is located 15 cm from the screen and has a focal length f2=+20 cm.
What should be the distance between the two lenses to obtain a sharp image on the screen?
  • 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