The question asks for the focal length of the lens. All parallel lines going through converging lens should pass through focal point, and light from a single source going through converging lens should exit in parallel rays...but the figure shows neither. I went with focal length =4cm as my choice but apparently you should use lensmaker's equation. If you just put the numbers in with 12 cm as object distance and 4cm as image distance you get 3cm focal length,...
Section Bank C/P Question 11