Solution

Suppose a light source is placed at the focal point of the optical system at $A$ which is at the distance $d$ from the left lens. The (virtual) image produced by the left lens must be at the focal point of the right lens at $B$ in order for parallel light rays to leave the right lens. Thus,

$$\frac{1}{f}=\frac{1}{d}-\frac{1}{f-l}\qquad\Rightarrow\qquad d=\frac{f(f-l)}{2f-l}$$

Consequently,

$$d=\frac{f(1-l/f)}{2(1-l/2f)}\approx\frac{f}{2}(1-\frac{l}{f})(1+\frac{l}{2f})\approx\frac{f}{2}-\frac{l}{4}.$$

Therefore, the focal length of the system is $d+l=\frac{f}{2}+\frac{3l}{4}$.