Practice Mental Math Strategies
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What you will learn
- 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 near-multiples of 10 and 100 mentally
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: break apart 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 the friendly number 20. Add 47 + 20 = 67. Subtract the extra 1: 67 - 1 = 66.
- To add 53 + 38: round 38 to 40. Add 53 + 40 = 93. Compensate by taking away the extra 2: 93 - 2 = 91.
- This works for near-multiples of 10 (like 19, 29, 39, 99).
Words to know
Mental math
Math you do in your head, without paper or a calculator
e.g. I used mental math to add 47 + 19.
Strategy
A smart way to solve a problem
e.g. My strategy is to break a number apart.
Friendly number
A round number that is easy to add or subtract
e.g. 20 is a friendly number that helps me solve 47 + 19.
Compensate
To add a little extra and then take it away
e.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 add
e.g. I break 19 into 10 + 9.
Choose the strategy that feels easiest for each problem. Practice each one until it is automatic.
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