7.5 min per walk around the block
        
                    
             
        
        
        
Answer:
x=5
Step-by-step explanation:
So how i came about to get this answer, was
y=15-3x
y=15-3(5)
y=15-15
y=0
I found five by testing out different numbers for x such as 1 2 3 4, but when i got to five, that was where i got the answer, as shown above. 
<u>Do</u><u> </u><u>not</u><u> </u><u>forget</u><u> </u><u>about</u><u> </u>PEMDAS
 
        
             
        
        
        
I hope this is the answer you want
 
        
        
        
Answer: 16............................