To answer this you can use a guess and check strategy.
Start with 5 minutes using it (25% lost) and 10 minutes charging (20% gained).
Try 7 minutes using it (35% lost) and 14 minutes charging (28%).
Next, 9 minutes (45%) and 18 minutes(36%).
10 minutes (50%) and 20 minutes (40%). This is the difference.
She used it for 10 minutes and charged it for 20 minutes.
        
                    
             
        
        
        
i believe it's 22.5 out of 45 to get exactly .500 
 
        
             
        
        
        
Ok so I heard this question a few times already and couldn't remember how it went so I decided to put my brain to the test, I used my algebra to use and complete it. 
If the Earth were shrunk down to the size of a basketball (radius:4.69 inches) the Moon would be a tennis ball (radius: 1.2768 inches, to be exact) 23.5 feet away.
The Sun would be a 42.6-foot sphere (source) located 1.7 miles away.
The closest star, Proxima Centauri, would still be 463,151 miles away – almost twice the distance of the actual moon." 
 
        
             
        
        
        
There is black white pants in my room