Teacher plz explain me this experiment .As I am not able to slove this

Solution : 

Such questions are made for enhancing the self creative skills and should be tried answering on your own. However, here are a few points that will help you in framing your answer:
• As given, the scale that is used here is a meter scale and after counting the divisions we can clearly tell that it is 8 m of length.
• Least count of the meter scale = 1 mm or 0.1 cm.
• Now, as given in the procedure we have to put the object both time in different position (some cm further or away from the initial reading) to get the actual position of the image.

• We know that, when the object is placed far away from the concave mirror or we can say at the infinite the image of the object is form at the focal point of the mirror.
• Lets take the distance of the object from the mirror is 70 cm and the screen is 50 cm in front of the mirror. 
u = - 70 cm
v = 50 cm
Using mirror formula 
1/f = 1/v + 1/ u
1/f = 1/50 + (-1/70)
1/f = 7 - 5/350
1/f = 2/350
f = 125 cm (f₁)
Similarly, we can also take another values and find the average focal length of the mirror, using the formula 
Average focal length = f_{​​​​​​1} + f₂/2

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