a. How much does the firm currently earn per share?
Since, the firm it's going to retain 90% of its earnings, then:
The firm earns $19.8 per share.
b. What proportion of the firm does currently own?
The firm has 40,000 shares and Sarah currently holds 400, so
Sarah owns 1% of the shares.
c. What proportion of the firm will Sarah own after the stock dividend?
Sarah will own 0.9% of the shares.
d. At what market price would you expect the stock to sell after the stock dividend?