TheThe  relative velocity of Deigo will be 27km/h . 
<h3>What is relative Velocity? </h3>
The relative velocity is the velocity of an object with respect to another observer. It is the time rate of relative position of one object with respect to another object . 
Vab = Va-Vb 
Where a is object and b is observer . 
We have given here the velocity of ball 20km/ h 
and the another velocity is 7km/ h 
Let us assume that Diego is also moving in the direction where ball is moving so the relative velocity  will  be 
  V = 20+7=27km/ h 
When both observer and object is moving in same direction than there sum of velocity will be relative velocity ,if they are moving opposite than subtraction. 
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Answer:
B. 24.2 m/s
Explanation:
Given;
mass of the roller coaster, m = 450 kg
height of the roller coaster, h = 30 m
The maximum potential energy of the roller coaster  due to its height is given by;



Therefore, the maximum speed of the roller coaster is 24.2 m/s.
 
        
             
        
        
        
Instead of asking this question go and study 
 
        
                    
             
        
        
        
I believe the answer is D
        
             
        
        
        
Since 1m/s=3.6 km/h, we can conclude that 10.0m/s = 36 km/h