Adding another card with value $25 would decrease the standard deviation
because it is closer to the mean the the current standard deviation.
        
                    
             
        
        
        
First isolate x by multiplying by x on both sides.  It will look like this:
(2/3)x=1.2
Then devise by (2/3) on both sides to get:
x= 1.2/(2/3)
The easiest way to get this is plug that into a calculator and get the answer:
x=1.8
 
        
             
        
        
        
Answer:
k=1 
Step-by-step explanation:

Subtract 8 from both sides 

Divide 

Simplify 
