Question

If a lens produces a real image that is four times as the object and is located 24cm from the lens, what is the
focal length of the lens?

Answer 1

$4=\frac{f}{d}\to d=\frac{24}{4}=6cm$

$\frac{1}{d}+\frac{1}{f}=\frac{1}{F}\to F=\frac{df}{d+f}=\frac{6\cdot24}{6+24}=4.8cm$