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Home > CC1MN > Chapter 9 > Lesson 9.3.4 > Problem 9-123



Juan got on an elevator at the middle floor of a building, went up floors, down floors, up floor, and down floors, where he left the elevator on the ground floor.

  1. How many floors are in the building?

    First, draw a vertical number line and pick a tick mark to represent the middle floor. A vertical number line with the middle tick mark, labeled, middle floor.

    Then trace his path, starting at the middle floor and move floors up. Movement of 4 up, from the middle floor.

    Then move floors down. Movement of 3 down, from the last position.

    Next, move floor up. Movement of 1 up, from the last position.

    Last, move floors down to the ground floor. Movement of 9 down, from the last position.

    If you stop on the first floor and there are seven floors below the middle floor, there should be floors above. How many floors are there? Numbers, 1 to 7, are labeled on each tick mark, starting with 1 from the last position.

  2. Explain how you found the number of floors.

    Did you work backwards?
    Or did you set up a number line?