Declaring variables-error

[TheSmartOne1]

I can't figure out why a gives a value of 0, Can anyone explain? Thanks in advance.
The output for this program is 770

include<stdio.h>
include<stdlib.h>
int a,b; 
void mystery(){ 
while(b>0){ 
int a; 
a=a+10; 
b=b-1; 
} 
}

int main(){ 
a=7; b=a; 
printf("%d\n",a); 
mystery(); 
printf("%d\n",a); 
printf("%d\n",b);

system("pause") 
return 0; 
}

[deceptikon]

Delete line 6, you're hiding the global variable with a local variable. Any changes to the local variable are lost on each iteration of the loop.