Sebastian was in a hotel lobby and took the elevator up 7 floors to his room. Then he took the elevator down 9 floors to the parking garage. He described his movement with the expression 9+ (-7). What is Sebastian's error?

Sebastian didn't interpret the numbers correctly (See explanation)

Step-by-step explanation:

If you go up an elevator, you are gaining floors, so the number is positive.

If you go down an elevator, you are losing floors, so the number is positive.

Looking at the statement, he went 7 floors up to his room, then 9 floors down to the parking garage this can be represented as , since the 7 is positive and the 9 is negative.

Sebastian described the movement as , which is wrong - 9 should be -9 and -7 should be 7.

Hope this helped!

Step-by-step explanation:

Let the lobby level be the reference point for Seb's movement.

The following describes his motion:  +7 - 9.  This puts him 2 floors below the lobby level.

Seb's "9 + (-7)" is incorrect because it indicates an upward trip, whereas the actual direction was downward.  Similarly, "-7" indicates a downward trip, whereas the action direction was upward.