when will the water reach the seventh rung

Enough of IIM and IAS Questions.. Now an IIT Question !!
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?

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As the Tide is raising the water
The Ship also get raised and hence its always the bottom rung which touches the water.