What is the apparent position of an object below a rectangular block of glass 6 cm thick, if a layer of water 4 cm thick is on the top of the glass?
(Take, 𝑛_𝑔𝑎 =1.5 and 𝑛_𝑤𝑎 =1.33).

here we can use the formula,
$\mathrm{\mu }=$ $\frac{\mathrm{real} \mathrm{depth}/\mathrm{thickness} \mathrm{of} \mathrm{the} \mathrm{object}}{\mathrm{apparent} \mathrm{depth}}$
Now due to refraction at two different boundaries the apparent depth of the object is,
here we can use the formula,
$\mathrm{\mu }=$$\frac{\mathrm{real} \mathrm{depth}/\mathrm{thickness} \mathrm{of} \mathrm{the} \mathrm{object}}{\mathrm{apparent} \mathrm{depth}}$
Now due to refraction at two different boundaries the apparent depth of the object is,
Apparent depth = $\frac{\text{ thickness of glass }}{{\mathrm{\mu }}_{\text{glass }}}+\frac{\text{ thickness of water }}{{\mathrm{\mu }}_{\text{waler }}}$
$=\frac{6}{1.5}+\frac{4}{1.3}=3+4=7\mathrm{cm}$