Ebb and Flow (example puzzle)

An epidemic of Snowpox has broken among the elves, but governor elf Pascaline is determined that the Christmas preparations should not be delayed. She has asked all around for a preventative against the contagious disease, and have finally received an answer from the Norwegian mountain trolls. These helpful trolls have volunteered to ship some of their secret super-herbal elixir to the North Pole. With this elixir, any healthy elf will be immunized for the entire season against the dreadful Snowpox.

On Elf Harbor, Pascaline has been awaiting the super-herbal elixir shipment, which was expected in the early morning. Now it is already 9:00 AM, and they are still waiting for the shipment. Impatiently, she paces around the dock, when she finds the supervisor of Elf Harbor, Heribert.

“Don’t worry, I’m in touch with the captain”, Heribert says. ”The ship has arrived but cannot enter the harbor because of the tides!”

“Of course, the tides!” Pascaline had completely forgotten about them. The ebb and flow of the tides causes the sea level to rise and fall, and the lower sea levels would be too shallow for the massive Norwegian ships.

“According to the trolls, the keel (the bottom of the ship) is 51.5 feet below the sea level,” continues Heribert, “and it needs an extra 1.5 feet between the keel and the ocean floor for safety. Here you can see the chart that shows today’s water level in the harbor according to any given time.”

Pascaline studies the tide information on the chart. The table shows that the average water level is 50 feet (15 meters).

What is the earliest time the ship can enter the harbor?

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a) Around 10:30 AM

b) Around 11:00 AM

c) Around 12:15 PM

d) Around 1:45 PM

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