Đáp án:

 \(60cm\)

Giải thích các bước giải:

a) Ta có: 

\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{{40}}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{{20}}{{OA' - 20}}\\
 \Rightarrow \dfrac{{40}}{{OA'}} = \dfrac{{20}}{{OA' - 20}} \Rightarrow OA' = 40cm
\end{array}\)

b) Ta có: 

\(\begin{array}{l}
\dfrac{{AB}}{{A'B'}} = \dfrac{{OA}}{{OA'}} = \dfrac{{40 - 15}}{{OA'}} = \dfrac{{25}}{{OA'}}\\
\dfrac{{AB}}{{A'B'}} = \dfrac{{OI}}{{A'B'}} = \dfrac{{OF'}}{{OA' - OF'}} = \dfrac{{20}}{{OA' - 20}}\\
 \Rightarrow \dfrac{{25}}{{OA'}} = \dfrac{{20}}{{OA' - 20}} \Rightarrow OA' = 100cm
\end{array}\)

Vậy khoảng cách giữa 2 ảnh là: \(100 - 40 = 60cm\)