Shareholders A.B. C. and D wish to enter into a shareholder's agreement, but they would like to see one provision before you draft the entire agreement. They want the agreement to provide that if any shareholder wants to sell shares to a third party, that shareholder must first offer the shares to the other shareholders, aka a right of first refusal. The shareholders were not good at explaining how to determine the number of shares each non selling shareholder could buy. They gave the following example:

Assume that the corporation has 50 shares issued and outstanding, and that A wants to sell the ten shares she owns. B, who owns 20 shares, could purchase 50% of those ten shares because B owns 50% of the shares not being sold.

Draft the formal for determine the number of shares that each non selling shareholder may purchase when a shareholder is selling all the shares it owns.