Grade 4 Mental Math Strategies Skip Counting: Children should review counting by 2’s, 5’s, 10’s, 25’s and 100’s. SCO: N3: Demonstrate an understanding of addition of numbers with answers to 10 000 and their corresponding subtractions (limited to 3 and 4-digit numerals) by • using personal strategies for adding and subtracting • estimating sums and differences • solving problems involving addition and subtraction Some strategies to consider: using benchmarks, rounding, front-end addition and subtraction (left-to-right calculations), and clustering of compatible numbers. benchmarks: 207 – 126 would give an answer between 75 (200 - 125), and 85 (210 - 125) rounding: 439 + 52 is approximately 440 + 50. front-end: 138 + 245 = 370 (200 + 100 is 300, 30 + 40 is 70 for an estimate of 370). Some students may include the ones in their estimate making their answer 380. front-end subtraction: 476 – 348 = 130 (400 – 300 is 100, 70 – 40 is 30, 6 and 8 are about the same so I’ll ignore them; my estimate is 130. clustering: cluster the 29, 35, and 42 together to make 100. compatibles: 225 + 68 + 75 = (225 + 75) + 68 = 368 See also Mental Math in the Middle Grades Lessons 10, 11, 13, 15 to 18, and 21 SCO:N5: Describe and apply mental mathematics strategies, such as: skip counting from a known fact using doubling or halving using doubling or halving and adding or subtracting one more group using patterns in the 9s facts using repeated doubling to determine basic multiplication facts to 9 × 9 and related division facts. doubling, e.g., for 4 3, think 2 3 = 6, so 4 3 = 6 + 6 doubling and adding one more group, e.g., for 3 7, think 2 7 = 14, and 14 + 7 = 21 use ten facts when multiplying by 9, e.g., for 9 6, think 10 6 = 60, and 60 – 6 = 54; for 7 9, think 7 10 = 70, and 70 – 7 = 63 halving, e.g., if 4 6 is equal to 24, then 2 × 6 is equal to 12 relating division to multiplication, e.g., for 64 ÷ 8, think 8 = 64. multiplying single-digit numbers by 10 and 100 SCO:N11: Demonstrate an understanding of addition and subtraction of decimals (limited to hundredths) by: • using compatible numbers • estimating sums and differences • using mental math strategies to solve problems. Use estimation strategies including: Compatible numbers: e.g., 0.72 + 0.23 are close to 0.75 and 0.25 which are compatible numbers so the sum of the decimal numbers must be close to 1. Front-end addition: e.g., 32.3 + 24.5 + 14.1; a student might think “30 + 20 + 10 is 60 and the ones and tenths clustered together make about another 10 for a total of 70.” Front-end subtraction: e.g., 4.76 – 3.48; a student might think “4 ones – 3 ones is 1 and 7 tenths – 4 tenths is 3 tenths for a difference of approximately 1 and 3 tenths.” Rounding: e.g., 4.39 + 5.2 is approximately 4+ 5 for an estimate of 9. Use mental computation strategies including: Compatible numbers: e.g., 3.55 + 6.45 or $3 and $6 would be $9 while 55 cents and 45 cents would make another dollar, for a sum of $10 or 10. Front-end strategy: e.g., 7.69 – 2. 45 A student might think “7 ones subtract 2 ones is 5 ones, 6 tenths subtract 4 tenths is 2 tenths and 9 hundredths subtract 5 hundredths is 4 hundredths, so the difference would be 5.24” Compensate: e.g., $4.99 + 1.98 + 0.99 could be calculated by finding the sum of 5 + 2 + 1 and then subtracting 0.04 or 4 cents. The sum would be $7.96. Counting on/ counting back: $2 – 1.48; a student might think, “2 more pennies would make $1.50 and 50 cents more makes $2 so the difference (change) is 52 cents Renaming: think of 3.2 + 0.9 as 32 tenths + 9 tenths.

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posted: | 3/28/2011 |

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