Case Study

Explain the student's mistake and write the correct expression.

Case context: A student translates 'nine less than a number xx' as 9−x9 - x.

Question: Explain the student's mistake and write the correct expression.

Sample answer: The student subtracted xx from 99 instead of 99 from xx. The correct expression is x−9x - 9.

Key points:

  • The phrase 'less than' means subtracting from the value that comes after it.
  • The starting value is xx, not 99.
  • The correct expression is x−9x - 9.

Rubric: Identifies that the subtraction order is reversed and provides the correct expression: x−9x - 9.

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Updated 2026-09-18

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