Question:

For combinations of two thin lenses are given in List I. The radius of curvature of all curved surfaces is r and the refractive index of all the lenses is 1.5. Match lens combinations in List I with their focal length in List II.

P-2, Q-4, R-3, S-1

P-1, Q-2, R-3, S-4

P-4, Q-1, R-2, S-3

P-2, Q-1, R-3, S-4

We use lens-makers' formula, 1/f = 1/f1 + 1/f2
Solving for a given set of lenses, we get
A → R/2
B → R
C → -R
d → 2R