Mental Math

lessonbunny.com

Answer Key & Explanations

Mental Math · Grade 3 · Worksheet 1

1. 698 + 275 = ? Answer: 973 Solution: Notice that 698 is very close to 700. Add 700 + 275 = 975. Since we added 2 extra (700 - 698 = 2), subtract 2 from the result.
   Full step-by-step solution

   Step 1: Notice that 698 is very close to 700. Step 2: Add 700 + 275 = 975. Step 3: Since we added 2 extra (700 - 698 = 2), subtract 2 from the result. Step 4: 975 - 2 = 973. The answer is 973.

2. Liam is collecting trading cards. He has 125 cards in his main collection and his friend Noah gives him 38 more cards. How many trading cards does Liam have in total now? Answer: 163 Solution: Liam starts with 125 trading cards. Noah gives him 38 more cards, so we need to add 38 to 125. Write the addition problem.
   Full step-by-step solution

   Liam starts with 125 trading cards. Noah gives him 38 more cards, so we need to add 38 to 125. Step 1: Write the addition problem. 125 + 38 Step 2: Add the ones place. 5 ones + 8 ones = 13 ones 13 ones means 1 ten and 3 ones. Write down the 3 in the ones place, and carry over the 1 ten to the tens place. Step 3: Add the tens place. 125 has 2 tens, plus 3 tens from 38, plus the 1 ten we carried over. 2 + 3 + 1 = 6 tens. Step 4: Add the hundreds place. 125 has 1 hundred, and 38 has no hundreds, so we keep 1 hundred. Step 5: Combine the digits. Hundreds: 1 Tens: 6 Ones: 3 So the total is 163. Therefore, Liam now has 163 trading cards in total.

3. Lily is baking cookies for her school bake sale. She needs to make 156 cookies and can fit 12 cookies on each baking tray. How many full trays of cookies will Lily need to bake? Answer: 13 Solution: Lily has 156 cookies total. Each tray holds 12 cookies. To find how many full trays she needs, divide 156 by 12.
   Full step-by-step solution

   Step 1: Lily has 156 cookies total. Step 2: Each tray holds 12 cookies. Step 3: To find how many full trays she needs, divide 156 by 12. Step 4: 12 × 13 = 156 Step 5: Since 12 × 13 equals exactly 156, Lily needs 13 full trays. The answer is 13.

4. 599 + 275 = ? Answer: 874 Solution: Notice that 599 is very close to 600. Add 600 + 275 = 875. Since we added 1 extra (600 - 599 = 1), subtract 1 from the result.
   Full step-by-step solution

   Step 1: Notice that 599 is very close to 600. Step 2: Add 600 + 275 = 875. Step 3: Since we added 1 extra (600 - 599 = 1), subtract 1 from the result. Step 4: 875 - 1 = 874. The answer is 874.

5. 798 + 125 = ? Answer: 923 Solution: Notice that 798 is very close to 800. Add 800 + 125 = 925. Since we added 2 extra (800 - 798 = 2), subtract 2 from the result.
   Full step-by-step solution

   Step 1: Notice that 798 is very close to 800. Step 2: Add 800 + 125 = 925. Step 3: Since we added 2 extra (800 - 798 = 2), subtract 2 from the result. Step 4: 925 - 2 = 923. The answer is 923.

6. Lily is helping her teacher organize books for the classroom library. She has 144 books to arrange equally on 8 shelves. How many books should Lily place on each shelf? Answer: 18 Solution: Lily has 144 books total. She needs to divide them equally among 8 shelves. To find how many books go on each shelf, divide 144 by 8.
   Full step-by-step solution

   Step 1: Lily has 144 books total. Step 2: She needs to divide them equally among 8 shelves. Step 3: To find how many books go on each shelf, divide 144 by 8. Step 4: 144 ÷ 8 = 18 Step 5: Check: 18 books per shelf × 8 shelves = 144 books total. The answer is 18.

7. 497 + 156 = ? Answer: 653 Solution: Notice that 497 is very close to 500. Add 500 + 156 = 656. Since we added 3 extra (500 - 497 = 3), subtract 3 from the result.
   Full step-by-step solution

   Step 1: Notice that 497 is very close to 500. Step 2: Add 500 + 156 = 656. Step 3: Since we added 3 extra (500 - 497 = 3), subtract 3 from the result. Step 4: 656 - 3 = 653. The answer is 653.

8. 398 + 275 = ? Answer: 673 Solution: Notice that 398 is very close to 400. Add 400 + 275 = 675. Since we added 2 extra (400 - 398 = 2), subtract 2 from the result.
   Full step-by-step solution

   Step 1: Notice that 398 is very close to 400. Step 2: Add 400 + 275 = 675. Step 3: Since we added 2 extra (400 - 398 = 2), subtract 2 from the result. Step 4: 675 - 2 = 673. The answer is 673.

9. Lily is planting flowers in her garden. She has 168 flower seeds and wants to plant them equally in 8 different flower beds. How many seeds should Lily plant in each flower bed? Answer: 21 Solution: Lily has 168 seeds total and wants to divide them equally among 8 flower beds. To find how many seeds go in each bed, divide 168 by 8. 168 ÷ 8 = 21 Check: 21 seeds per bed × 8 beds = 168 seeds total.
   Full step-by-step solution

   Step 1: Lily has 168 seeds total and wants to divide them equally among 8 flower beds. Step 2: To find how many seeds go in each bed, divide 168 by 8. Step 3: 168 ÷ 8 = 21 Step 4: Check: 21 seeds per bed × 8 beds = 168 seeds total. The answer is 21.