An object is placed on the principal axis of convex lens of focal length $10 cm$ as shown A plane mirror is placed on the other side of lens at a distance of $20 cm$ The image produced by the plane mirror is $5 cm$ inside the mirror The distance of the object from the lens is ___$cm$
The complex number $z=\frac{i-1}{\cos \frac{\pi}{3}+i \sin \frac{\pi}{3}}$ is equal to :
Among the relations $S=\left\{(a, b): a, b \in R -\{0\}, 2+\frac{a}{b}>\right\}$ and $T=\left\{(a, b): a, b \in R , a^2-b^2 \in Z\right\}$,
Let the mean and standard deviation of marks of class A of $100$ students be respectively $40$ and $\alpha$ (> 0 ), and the mean and standard deviation of marks of class B of $n$ students be respectively $55$ and 30 $-\alpha$. If the mean and variance of the marks of the combined class of $100+ n$ students are respectively $50$ and $350$ , then the sum of variances of classes $A$ and $B$ is :