Given,
f(x)=[a+13sinx],x∈(0,π)
⇒f(x)=[13sinx]+a, as a∈Integer
Now let g(x)=13sinx, now plotting the diagram of y=13sinx we get,

Now from above diagram we can see that,
f(x)=[13sinx] is not differentiable at intersection points of y=k,k\in [1,13]&k\in Z
⇒Points of non-differentiability=12×2+1=25