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The parallel rays (to principal axes)  after being incident on the convex lens tend to converge at the focus of the lens. 

There a plane mirror between the lens and its focus.  The rays get reflected with the same angles of incidence and reflection.  The plane mirror does not change the converging distance of the rays.

The path travelled by the rays after refraction from the lens = 10 cm + 10 cm = 20 cm

Focal length = 20 cm
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