Mental Math Within 20

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Answer Key & Explanations

Mental Math Within 20 · Grade 2 · Worksheet 2

1. 63 - 28 = ? Answer: 35 Solution: Write the problem vertically: 63 - 28 Look at the ones column: 3 - 8. Since 3 is smaller than 8, we need to regroup from the tens column.
   Full step-by-step solution

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

2. Charlotte has a number path that shows jumps of 10. She starts at 27 and jumps forward 3 times. What number does she land on? Answer: 57 Solution: Charlotte starts at 27. She makes 3 jumps forward, each of 10. 10 + 10 + 10 = 30, so she adds 30 in total.
   Full step-by-step solution

   Step 1: Charlotte starts at 27. Step 2: She makes 3 jumps forward, each of 10. Step 3: 10 + 10 + 10 = 30, so she adds 30 in total. Step 4: 27 + 30 = 57. The answer is 57.

3. 75 + 26 = ? Answer: 101 Solution: Add the ones digits: 5 + 6 = 11 Write down 1 in the ones place and carry over 1 to the tens column Add the tens digits: 7 + 2 + 1 (carried over) = 10 Write down 10 in the tens and hundreds places The final answer is 101
   Full step-by-step solution

   Step 1: Add the ones digits: 5 + 6 = 11 Step 2: Write down 1 in the ones place and carry over 1 to the tens column Step 3: Add the tens digits: 7 + 2 + 1 (carried over) = 10 Step 4: Write down 10 in the tens and hundreds places Step 5: The final answer is 101

4. 72 - 28 = ? Answer: 44 Solution: Start with 72 - 28. Look at the ones place: 2 - 8. Since 2 is less than 8, we need to borrow from the tens place.
   Full step-by-step solution

   Step 1: Start with 72 - 28. Step 2: Look at the ones place: 2 - 8. Since 2 is less than 8, we need to borrow from the tens place. Step 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). Step 4: Now subtract the ones: 12 - 8 = 4. Step 5: Now subtract the tens: 6 (from the borrowed 7) - 2 = 4. Step 6: Combine the tens and ones: 40 + 4 = 44. The answer is 44.

5. Lily has 45 crayons in her art box. Her teacher gives her 28 more crayons for a class project. How many crayons does Lily have now? Answer: 73 Solution: Start with Lily's original crayons: 45 Add the new crayons: 45 + 28 Add the tens: 40 + 20 = 60 Add the ones: 5 + 8 = 13 Combine tens and ones: 60 + 13 = 73 The answer is 73.
   Full step-by-step solution

   Step 1: Start with Lily's original crayons: 45 Step 2: Add the new crayons: 45 + 28 Step 3: Add the tens: 40 + 20 = 60 Step 4: Add the ones: 5 + 8 = 13 Step 5: Combine tens and ones: 60 + 13 = 73 The answer is 73.

6. 46 + 31 = ? Answer: 77 Solution: Break 46 into 40 + 6 Break 31 into 30 + 1 Add the tens: 40 + 30 = 70 Add the ones: 6 + 1 = 7 Combine the results: 70 + 7 = 77 The answer is 77.
   Full step-by-step solution

   Step 1: Break 46 into 40 + 6 Step 2: Break 31 into 30 + 1 Step 3: Add the tens: 40 + 30 = 70 Step 4: Add the ones: 6 + 1 = 7 Step 5: Combine the results: 70 + 7 = 77 The answer is 77.

7. Kaia is collecting seashells. Kaia has 23 shells. Kaia finds 32 more on the beach. How many shells does Kaia have in all? Answer: 55 Solution: Start with 23 shells. Find 32 more: 23 + 32 = 55. Kaia has 55 shells in all.
   Full step-by-step solution

   Step 1: Start with 23 shells. Step 2: Find 32 more: 23 + 32 = 55. Kaia has 55 shells in all.

8. Moana and Moana are playing a card game. Moana has 36 points. Moana needs 27 more points to win. How many points does a player need to win the game? Answer: 63 Solution: Moana has 36 points. Add the points needed: 36 + 27 = 63. A player needs 63 points to win.
   Full step-by-step solution

   Step 1: Moana has 36 points. Step 2: Add the points needed: 36 + 27 = 63. A player needs 63 points to win.

9. Noah has 32 toy cars in his collection. For his birthday, he received 25 more toy cars. How many toy cars does Noah have now? Answer: 57 Solution: Noah started with 32 toy cars He received 25 more toy cars Add the two numbers together: 32 + 25 Add the ones place: 2 + 5 = 7 Add the tens place: 30 + 20 = 50 Combine the results: 50 + 7 = 57 Noah has 57 toy cars now.
   Full step-by-step solution

   Step 1: Noah started with 32 toy cars Step 2: He received 25 more toy cars Step 3: Add the two numbers together: 32 + 25 Step 4: Add the ones place: 2 + 5 = 7 Step 5: Add the tens place: 30 + 20 = 50 Step 6: Combine the results: 50 + 7 = 57 Noah has 57 toy cars now.

10. Matiu drew a number path that starts at 46. He jumps backward 3 tens. What number does he land on? Answer: 16 Solution: Start at 46. 46 has 4 tens and 6 ones. Jumping backward 3 tens means subtracting 30: 46 - 30.
    Full step-by-step solution

    Step 1: Start at 46. Step 2: 46 has 4 tens and 6 ones. Step 3: Jumping backward 3 tens means subtracting 30: 46 - 30. Step 4: Subtract the tens: 4 tens - 3 tens = 1 ten. Step 5: The ones stay the same: 6 ones. Step 6: 1 ten and 6 ones is 16. The answer is 16.