Optics

Answer is A
This is combination of lens so we are supposed to apply lens formula
But the backside solution reveal that it must be mirror formula

@PratikPatil
Whenever a part of combination of lenses is silvered. ...it is to be treated as a mirror.....now there is a formula to calculate the focal length of such combination ...but at the end the combination is always treated as a mirror...same in this case also......on applying mirror formula directly for given data we get the answer