Answer:
45.04 cm
Explanation:
The tired physics professor red easily at a distance of 26 cm
So v = 26 cm
And it is given that object distance that is u = 61.5 cm
There is relation between focal length object distance and image distance that is 
Here f =focal distance v = image distance and u = object distance
So 
So f=45.04 cm
The time required by the car to stop is 4.916 sec.
Since the car is moving with the constant deceleration we can apply the first equation of motion to calculate the time required by the car to stop.
The first equation of motion is given as
V=u+at
Here, V=final speed of the car=0 mi/h as the car stops
u =initial speed of the car=55 mi/hr=24.58 m/s
a= acceleartion =-5 m/s^2 (here negative sign indicates for deceleration)
Now applying the values in the first equation
V=u+at
0=24.58-5*t
t=4.916 sec
Therefore the car will stops in 4.916 sec.
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