🔢 MathGrade 3Lesson 10
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Practice Mental Math Strategies
Lesson preview: the Mini Lesson and Vocabulary are open to everyone. Guided practice, exercises, the parent checklist and the printable PDF worksheet come with full access (free 7-day trial or membership).
Learning Objectives
- 1Apply the make-10 strategy to add single-digit numbers mentally
- 2Use doubles and near-doubles to solve addition facts quickly
- 3Round-and-adjust to add or subtract near-multiples of 10 and 100 mentally
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Mini Lesson
Mental math means calculating in your head without writing anything down. Using strategies makes this faster and easier. The three main strategies are: make-10, doubles (and near-doubles), and round-and-adjust.
Make-10 Strategy
- To add 8 + 5: split 5 into 2 + 3. Then 8 + 2 = 10, and 10 + 3 = 13.
- You choose the split so that one part completes 10 with the first number.
- This works well whenever one addend is close to 10 (like 7, 8, or 9).
Doubles and Near-Doubles
- Doubles: 6 + 6 = 12. Learn these facts by heart.
- Near-doubles: 6 + 7 = 6 + 6 + 1 = 12 + 1 = 13.
- Or: 6 + 7 = 7 + 7 - 1 = 14 - 1 = 13.
Round-and-Adjust Strategy
- To add 47 + 19: round 19 to 20. Add 47 + 20 = 67. Subtract the extra 1: 67 - 1 = 66.
- To add 53 + 38: round 38 to 40. Add 53 + 40 = 93. Subtract 2: 93 - 2 = 91.
- This works for near-multiples of 10 (like 19, 21, 29, 39, 99, 101).
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Choose the strategy that feels easiest for each problem. Practice each one until it is automatic.
Vocabulary
Mental math
Math you do in your head, without paper or a calculatore.g. I used mental math to add 47 + 19.
Strategy
A smart way to solve a probleme.g. My strategy is to break a number apart.
Friendly number
A round number that is easy to add or subtracte.g. 50 is a friendly number that helps me solve 47 + 19.
Compensate
To add a little extra and then take it awaye.g. To add 47 + 19, I do 47 + 20 = 67, then take away 1.
Break apart
To split a number into smaller parts that are easier to adde.g. I break 19 into 10 + 9.
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