In this q. Everything is clear but i can't understand the last part, why 15 was subtracted? I mean how it tells that the image will coincide??please explain this part.

21. If an object is held at a distance of 60 cm from a convex mirror of focal length 20 cm. at what distance from the convex mirror, would a plane mirror be held, so that images in the two mirrors  coincide?

Dear Student ,

From both of the equation we got 
BM= 37.5
as both are equal to BM 
so x+15 = 37.5 
so x = 37.5-15 
x=22.5 cm 

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