Answer:
0.125 cm
Explanation:
1/f = 1/d¡ + 1/d。
Find the focal point 
(13.0^-1 + 20.8^-1) = 0.125 m 
Focal point = 0.125 m  
 
        
                    
             
        
        
        
Answer: 
The force of friction acting on block B is approximately 26.7N.  Note: this result does not match any value from your multiple choice list. Please see comment at the end of this answer.   
Explanation: 
The acting force F=75N pushes block A into acceleration to the left. Through a kinetic friction force, block B also accelerates to the left, however, the maximum of the friction force (which is unknown) makes block B accelerate by 0.5 m/s^2 slower than the block A, hence appearing it to accelerate with 0.5 m/s^2 to the right relative to the block A.
To solve this problem, start with setting up the net force equations for both block A and B:

where forces acting to the left are positive and those acting to the right are negative. The friction force F_fr in the first equation  is due to A acting on B and in the second equation due to B acting on A. They are opposite in direction but have the same magnitude (Newton's third law). We also know that B accelerates 0.5 slower than A:

Now we can solve the system of 3 equations for a_A, a_B and finally for F_fr:

The force of friction acting on block B is approximately 26.7N. 
This answer has been verified by multiple people and is correct for the provided values in your question. I recommend double-checking the text of your question for any typos and letting us know in the comments section.
 
        
                    
             
        
        
        
 The velocity of the ball when it was caught is 12.52 m/s.
<em>"Your question is not complete it seems to be missing the following, information"</em>,
find the velocity of the ball when it was caught.
The given parameters;
maximum height above the ground reached by the ball, H = 38 m
height above the ground where the ball was caught, h = 30 m
The height traveled by the ball when it was caught is calculated as follows;
y = H - h
y = 38 - 30 = 8 m
The velocity of the ball when it was caught is calculated as;

Thus, the velocity of the ball when it was caught is 12.52 m/s.
Learn more here: brainly.com/question/14582703
 
        
             
        
        
        
Abraham Lincoln 
Explanation:
hope it helps you a little