Mar 2, 2018

See below.

#### Explanation:

For a biconvex lens we generally take $f = \frac{R}{2}$ (by generally i'm actually refering to a 10th gradier in India like me and ujjwal who has asked this question)

Now, using lens maker formula

$\frac{1}{f} = \left(n - 1\right) \left(\frac{1}{R} _ 1 - \frac{1}{R} _ 2\right)$

In case ${R}_{1} = {R}_{2}$
The formula becomes

$\frac{1}{f} = \left(n - 1\right) \left(\frac{2}{R}\right)$

Generally, we are too ignorant about n (refractive index) and we eliminate $\left(n - 1\right)$ as 1, this is what we have to do here.

Here $n - 1 = 1$ , you have to accept it for time being because of absence of proper details.

$\frac{1}{f} = \left(\frac{1}{R} _ 1 - \frac{1}{R} _ 2\right)$ $\text{ }$ (${R}_{2} = - \infty$)

$\frac{1}{f} = \frac{1}{R}$

Now recall the ray diagrams , if object is at C the image will also be at C.

Now, since there is a mirror so to form image of object on object itself the object distance should be $10 c m$.
Answer $= 10 c m$