Answer to Question 1
I would suggest that the firm hire the third worker. The marginal productivity of the extra ten workers is 800 gallons of water per hour. This translates into 80.00 of MRP (800 x 10 cents). Since the firm is paying 7 per worker the marginal cost of hiring the ten additional workers if 70.00 . This makes it profitable for the firm to hire them.
Answer to Question 2
I would suggest that the firm not hire the fifth worker because his marginal revenue product will be 8 (40 0.20) and this is less than the market wage rate. Therefore, the firm would lower its profits if it hired the fifth worker.