A ladder hangs over the side of a ship anchored in a port. The bottom rung touches the water. The distance between rungs is 20 cm and the length of the ladder is 180 cm. The tide is rising at the rate of 15 cm each hour.
When will the water reach the seventh rung from the top?
If the tide is raising water, then it is raising the ship on water, too. So water will reach still the first rung.