A box contains \(6\) red marbles, \(5\) blue marbles, \(2\) yellow marbles, and \(3\) green marbles. Several orange marbles are added to the box. All the marbles in the box are identical except for colour.
A marble is then randomly selected from the box, and the probability that a blue or green marble is selected is \(\tfrac{2}{7}\).
How many orange marbles were added to the box?
Themes: Algebra, Data Management