Teaching mental addition and subtraction of 10 to 3-digit numbers

Question

Hey there! 👋 Ever struggled with adding or subtracting 10 in your head? It's super useful for everyday stuff like calculating change or figuring out how much time you have left. Let's make it easy! 😃

heather997 · Accepted Answer

📚 Objectives

🎯 Students will be able to mentally add 10 to any 3-digit number.
  🧠 Students will be able to mentally subtract 10 from any 3-digit number.
  ⏱️ Students will improve their mental calculation speed and accuracy.
  👍 Students will apply these skills in problem-solving scenarios.

🧮 Materials

whiteboard or projector.
   markers or pens.
   number cards (100-999).
   worksheets with practice problems.

🚀 Warm-up (5 minutes)

🗣️  Quick review of place value (hundreds, tens, ones).
  ➕ Practice adding and subtracting single-digit numbers mentally.
  🔢 Count forward and backward by 10s from various starting points (e.g., 23, 156, 389).

👨‍🏫 Main Instruction (20 minutes)

Adding 10:
    
      ➕ Explain that adding 10 only affects the tens place.
      ✍️ Demonstrate with examples:
        
          $123 + 10 = 133$
          $256 + 10 = 266$
          $391 + 10 = 401$

🗣️ Emphasize that if the tens place is 9, adding 10 will increase the hundreds place and reset the tens place to 0.

Subtracting 10:
    
      ➖ Explain that subtracting 10 only affects the tens place.
      ✍️ Demonstrate with examples:
        
          $456 - 10 = 446$
          $582 - 10 = 572$
          $603 - 10 = 593$

🗣️ Emphasize that if the tens place is 0, subtracting 10 will decrease the hundreds place and set the tens place to 9.

Guided Practice:
    
      🤝 Work through several examples together as a class.
      🙋 Encourage students to explain their thought processes.
      ✏️ Use number cards to generate random 3-digit numbers and have students add or subtract 10 mentally.

📝 Assessment (10 minutes)

✍️ Distribute a worksheet with a mix of addition and subtraction problems involving adding/subtracting 10 from 3-digit numbers.
  ⏱️ Set a time limit to encourage mental calculation.
  ✅ Review answers as a class and address any misconceptions.

➕ Practice Quiz

➕ What is $234 + 10$?
  ➖ What is $378 - 10$?
  ➕ What is $492 + 10$?
  ➖ What is $511 - 10$?
  ➕ What is $655 + 10$?
  ➖ What is $701 - 10$?
  ➕ What is $829 + 10$?