Mental Math Within 20

Grade 2 · mathematics · 86 practice problems · read aloud

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🧠 Mental Math Within 20

What is it and why is it useful?

Mental math is solving problems in your head without using paper, fingers, or a calculator. Being able to quickly add and subtract numbers up to 20 helps you in everyday life, like when you're sharing snacks, counting game points, or working faster in class! 🚀

How to Solve Problems

  1. Look at the problem: Is it addition (+) or subtraction (-)?
  2. Find a friendly number: Look for a 10. Numbers that make 10 are your friends! (Like 8+2 or 7+3).
  3. Break numbers apart: For a tricky problem like 9+5, think "9 needs 1 more to make 10." So, take 1 from the 5 to make 10, then add the leftover 4 to get 14.
  4. Check your answer: Does it make sense? If you added, your answer should be bigger than the numbers you started with.

Visual Examples

Example 1: 8 + 6

Step 1: 8 needs 2 to make 10.
Step 2: Take 2 from the 6, so 6 = 2 + 4.
Step 3: 8 + 2 = 10.
Step 4: 10 + 4 = 14.
So, 8 + 6 = 14. ✅

Example 2: 15 - 7

Step 1: Think "What goes with 7 to make 15?"
Step 2: 7 + 3 = 10.
Step 3: 10 + 5 = 15.
Step 4: Add the parts you used: 3 + 5 = 8.
So, 15 - 7 = 8. ✅

Common Mistakes to Avoid

Mixing up + and -: Always check the sign first! Addition makes numbers bigger, subtraction makes them smaller.

Forgetting to make a 10: Using the "make a 10" strategy is a superpower for solving these problems quickly and correctly.

Finger counting for everything: Try to picture the numbers in your head. It gets faster with practice!

Tips & Tricks

  • Doubles are easy! Memorize doubles like 6+6=12 and 8+8=16.
  • Near Doubles: If you know 7+7=14, then 7+8 is just one more, so 15!
  • +9 Shortcut: Adding 9 is like adding 10 and then taking away 1. 13+9 is the same as 13+10=23, then 23-1=22.
  • Fact Families: If 9+4=13, then you also know 4+9=13, 13-9=4, and 13-4=9.

How to Practice

Practice for just 5-10 minutes each day!

  • Flashcards: Make your own or use an app.
  • Math Games: Play "war" with a deck of cards or find fun math games online.
  • Real-Life Problems: "If I have 11 grapes and eat 4, how many are left?"
  • Chanting: Say the answers to problems out loud until they stick in your memory.

Practice problems

6 of the 86, worked through step by step — try them before opening the answer.

1 78 - 45 = ?

Hint: Think about subtracting the tens first, then the ones.

Show the answer

Answer: 33

  1. Start with 78.
  2. Subtract the tens: 70 - 40 = 30.
  3. Subtract the ones: 8 - 5 = 3.
  4. Combine the results: 30 + 3 = 33.

The answer is 33.

2 72 - 28 = ?

Hint: Think about subtracting the ones place first, then the tens place. If you need to borrow, remember that 1 ten equals 10 ones.

Show the answer

Answer: 44

  1. Start with 72 - 28.
  2. Look at the ones place: 2 - 8. Since 2 is less than 8, we need to borrow from the tens place.
  3. Borrow 1 ten from the 7 in the tens place. This changes the 7 to a 6, and gives 12 ones (10 + 2 = 12).
  4. Now subtract the ones: 12 - 8 = 4.
  5. Now subtract the tens: 6 (from the borrowed 7) - 2 = 4.
  6. Combine the tens and ones: 40 + 4 = 44.

The answer is 44.

3 63 - 28 = ?

Hint: Think about subtracting tens and ones separately, and remember to regroup if needed.

Show the answer

Answer: 35

  1. Write the problem vertically: 63 - 28
  2. Look at the ones column: 3 - 8. Since 3 is smaller than 8, we need to regroup from the tens column.
  3. Take 1 ten from the 6 tens, which becomes 5 tens, and add 10 ones to the 3 ones, making 13 ones.
  4. Now subtract the ones: 13 - 8 = 5
  5. Subtract the tens: 5 - 2 = 3
  6. Combine the results: 3 tens and 5 ones = 35
  7. Check: 35 + 28 = 63, which confirms our answer is correct.

The answer is 35.

4 63 + 28 = ?

Hint: Add the ones digits first, then the tens digits. Remember to carry over if needed.

Show the answer

Answer: 91

  1. Add the ones digits: 3 + 8 = 11. Write down 1 in the ones place and carry over 1 to the tens column.
  2. Add the tens digits including the carry-over: 6 + 2 + 1 = 9. Write down 9 in the tens place.
  3. Combine the tens and ones: 91.

The answer is 91.

5 68 + 24 = ?

Hint: Add the ones digits first, then the tens digits. Remember to carry over if needed.

Show the answer

Answer: 92

  1. Add the ones digits: 8 + 4 = 12. Write down 2 in the ones place and carry over the 1 to the tens column.
  2. Add the tens digits: 6 + 2 = 8, plus the carried over 1 makes 9.
  3. Combine the tens and ones: 90 + 2 = 92.

The answer is 92.

6 64 + 28 = ?

Hint: Add the ones digits first, then add the tens digits. Remember to carry over if needed.

Show the answer

Answer: 92

  1. Add the ones digits: 4 + 8 = 12. Write down 2 in the ones place and carry over 1 to the tens place.
  2. Add the tens digits: 6 + 2 = 8, plus the carried over 1 makes 9.
  3. Combine the tens and ones: 90 + 2 = 92.

The answer is 92.

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