You want a large bowl of salad to share with your friends. your brother eats 1/3 of it before they came over. what fractional portion of the original bowl of salad does each friend receive?

If you have a large bowl of salad (which is a whole 1) and your brother eats 1/3 of it before your friends come over, then you have:

1 - 1/3 = 2/3 of salad left

We don't know how many friends will come, so let's assume it's x of them who will visit you. In such case we have to divide 2/3 by x friends:

2/3 ÷ x =
= 2/3 * 1/x =
= 2/3x

Each friend will get 2/3x of the salad.