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A student solved and wrote . What mistake did the student make?

Class 7Simple EquationsSimple Equations
  • ASubtracted 5 instead of adding 5 to both sides
  • BDivided instead of subtracting
  • CForgot to write the variable
  • DNo mistake; is correct

Step-by-Step Solution

To undo '− 5' you must add 5: . The student wrongly subtracted, getting 4.
Answer: A) Subtracted 5 instead of adding 5 to both sides

Key idea: Transposition — the shortcut

When you're comfortable with the balance method, you can move terms across the '=' sign in one step by **flipping the sign** of the term that moves.


→ move +2 across, flipping it to −2:
→

For the multiplier: move ×3 across as ÷3 (and ÷ moves across as ×). Same balance idea, faster notation. **The sign flips because addition and subtraction are inverses, multiplication and division are inverses.**

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