Consider the situation shown in figure. Water$\left({\mu }_W=\frac{4}{3}\right)$  is filled in a breaker up to a height of 10 cm. A plane mirror fixed at a height of 5 cm from the surface of water. Distance of image from the mirror after reflection from it of an object O at the bottom of the beaker is:

From figure it is clear that object appears to be raised by $\frac{10}{4}cm\left(2.5cm\right)$

Hence distance between mirror and O'= 5 + 7.5 = 12.5cm

So, final image will be formed at 12.5 cm behind the plane mirror.

*** ALTERNATE METHOD  : 

for plane mirror object distance =image distance

                  v = 5 + $\frac{10}{4/3}= 12.5$ cm