Explain this one and give me correct solution

The angle of depression of a ship viewed at a particular instant from the top of a 75 m vertical cliff is 30. Find the distance of the ship from the base of the cliff at this instant. The ship is sailing away from the cliff at constant speed and 1 minute later its angle of depression from the top of the cliff is 20. Determine the speed of the ship in km/h Figure 21.31 shows the cliff AB, the initial position of the ship at C and the final position at D. Since the angle of depression is initially 30, ZACB = 30 (alternate angles between parallel lines). - 30 . 20 75 m 307 20 B D Figure 21.31