From the top of a 150 m high cliff, the angle of depression of a boat sailing straight towards the cliff changes from 30° to 45° in 2 minutes. The speed of the boat is:

The boat's distances from the foot of the cliff are 150/tan30° = 150√3 m and 150/tan45° = 150 m. It covers 150√3 − 150 = 150(√3 − 1) m in 2 minutes, so the speed is 75(√3 − 1) m/min ≈ 54.9 m/min.