\(30\) से.मी. फोकस दूरी के उत्तल लेन्स से \(60\) से.मी. दूरी पर एक बिन्दु वस्तु उपस्थित है। यदि एक समतल दर्पण, मुख्य अक्ष के लम्बवत् तथा इससे \(40\) से.मी. दूरी पर रखा जाता है, तो अन्तिम प्रतिबिम्ब पाया जायेगा दूरी :

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.