What Is the Refractive Index of the Glass Slab?

[sachin123]

Homework Statement

Please see here:
http://img198.imageshack.us/i/unledev.jpg/

object placed 8 cm before the slab(6 cm thick,silvered at other end).
The final image is formed 10 cm behind the mirror.Find refractive index of slab.

2. The attempt at a solution

n is refractive index
Okay.
So the object 'sees' the mirror at a smaller distance due to normal shift.
it sees the mirror at a distance:

8 + 6- 6/n cm

so the image as seen by the object will be at twice the object distance,
or
at 8 + 6- 6/n cm from the apparent position of the mirror.
It is given that the image is formed at a distance from 10 cm behind the actual mirror.
So,
I get to equate,
8 + 6- 6/n and 10 + 6/n.

so 8 + 6- 6/n =10 + 6/n.
which leads me to n=3.
But the answer is 1.5?

What mistake have I made?Please help me get the concept right.
This mistake keeps happening to me
I appreciate any help

 

[sachin123]

At least please tell me what I should be doing if you didn't understand my working.