In the situation shown coefficient of friction between A and B is 0.5 and between B and C is 0.3 . Friction acting between B and C is xN then `(9x)/7` is

Explanation

Let everything moves together
Then,

`a=(12)/(12)=1 m//s^(2)`

But `f_(AB "maximum")=15N`
So, sliding occurs.
Now, see if `B` and `C` move together

`a=(15-8)/(9)=(7)/(9)m'//s^(2)`
So, friction acting between `B` and `C` is `(7)/(9)xx5 m//s^(2)`