The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of $2 \mathrm{m}/\mathrm{s}$. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be

In a plane mirror, the object distance and the image distance from the mirror always remain the same. 

So, when the car reverses at a speed of ${\mathrm{s}}_1 = 2 \mathrm{m}/\mathrm{s}$, then its image also appears to move towards the mirror at the same speed.

 i.e., ${\mathrm{s}}_2 = {\mathrm{s}}_1 = 2 \mathrm{m}/\mathrm{s}$.

Hence, the speed at which the image of the truck will appear to approach the car will be 

$\mathrm{s} ={\mathrm{s}}_1 + {\mathrm{s}}_2$

$\mathrm{s} =2 \mathrm{m}/\mathrm{s} + 2 \mathrm{m}/\mathrm{s}$

$\mathrm{s} =4 \mathrm{m}/\mathrm{s}.$