Задача 40503.

The subject is at a distance L = 0.9 m from the screen. Between the subject and the screen moves the lens, with a lens at one position on the screen is an enlarged image of the object, while the other - reduced. Is the focal length F of the lens, if the linear dimensions of the first image to k = 4 times larger than the second?

Detailed solution shorthand terms, formulas and laws used in the decision, the withdrawal of the calculation formula and the response.
If you have any questions to address, please. I´ll try to help.