You make a large bowl of salad to share with your friends.Your brother eats 1/3 of it before they come.What fractional portion of the origanal bowl of salad does each friend receive
(2/3) / 7
(2/3) * (1/7) = 2/21
That's assuming you are eating the same amount as your friends.
If not -- then each will get 1/9 of the original bowl of salad.