A ring is placed at the bottom of a trough containing four immiscible liquids of refractive indices 1.0, 2.0, 3.0 and 4.0 poured one above the other…

A ring is placed at the bottom of a trough containing four immiscible liquids of refractive indices $\displaystyle 1.0$, $\displaystyle 2.0$, $\displaystyle 3.0$ and $\displaystyle 4.0$ poured one above the other of heights $\displaystyle 10$ cm, $\displaystyle 20$ cm, $\displaystyle 30$ cm, and $\displaystyle 40$ cm respectively. The apparent depth at which the ring appears, when seen from outside is \_\_\_\_ cm .