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 [tex]\frac{1}{f}=\frac{1}{v}-\frac{1}{u}[/tex]
Here f =focal distance v = image distance and u = object distance
So [tex]\frac{1}{f}=\frac{1}{26}-\frac{1}{61.5}[/tex]
So f=45.04 cm