Mark gives 8 cookies to his friends and have 6 cookies left. he gives 9 cookies to each friend and have 1 cookie left. how many friends does he have?

Mark has 5 friends. 

In order to find this you need to set up a system of equations. Let x equal the amount of cookies Mark has. Let y be the amount of friends that Mark has. 

The first equation will show the first scenario. 
x - 8y = 6

The second equation will show the second scenario. 
x - 9y =1

Now we can solve the system of equations through addition. Multiply the second equation by -1 and then add them together (the x values will cancel out). 

x - 8y = 6
-x + 9y = -1

y = 5

Since y = number of friends, we know he has 5 friends.