Each friend ate 3/7 of a carton of ice cream.
Since we know that 7 friends ate 3 cartons of ice cream, where each friend ate an equal amount, and we want to know how much each friend ate, we can set up a proportion to find x. A valid proportion would be:
7/3 = 1/x
where the numerators of both ratios show the number of friends while the denominators of both ratios show the amount of ice cream eaten by the number of friends in the numerator. Then, we can cross multiply, allowing us to come up with the equation 7x = 3. Then, we can solve this one-step algebraic equation and learn that x = 3 / 7. Therefore, each friend of the 7 friends ate 3/7 of a carton of ice cream.