Mental Math
Which pair makes an easy whole number: 12.35 + 7.65 or 12.35 + 7.35?
12.35 + 7.65 = 20.00. The decimals combine to make exactly 1 whole.
Estimate by rounding to whole numbers: 4.6 + 3.2.
5 + 3 = about 8.
What does each decimal block represent: a flat, a rod, and a small square?
Flat = 1 whole. Rod = 0.1. Small square = 0.01.
Find the sum: 3.29 + 0.88.
4.17. Line up the decimal points and regroup.
A poster costs $12.45. Another costs $7.04. How much more does the first poster cost?
$12.45 - $7.04 = $5.41.
Use compensation to solve 6.97 + 4.15.
7.00 + 4.15 = 11.15. Then subtract the extra 0.03. The answer is 11.12.
Estimate by rounding to whole numbers: 12.48 - 5.09.
12 - 5 = about 7.
Use place value to add 0.55 + 0.29.
0.84. Regroup 10 hundredths as 1 tenth.
Dave sold 14.84 pounds of cookies. Theo sold 21.45 pounds. How many pounds altogether?
14.84 + 21.45 = 36.29 pounds.
A runner's goal is 15 miles. She ran 2.7, 3.6, and 4.3 miles. How many miles are left?
2.7 + 3.6 + 4.3 = 10.6. Then 15 - 10.6 = 4.4 miles.
Find the sum mentally: 23.85 + 7.42 + 16.15.
23.85 + 16.15 = 40.00. Then 40.00 + 7.42 = 47.42.
Which is the best estimate for 21.39 + 21.59: 32, 43, or 53?
43. Round to 21 + 22.
Use place value to subtract 1.48 - 0.83.
0.65. Regroup 1 whole as 10 tenths.
Three shirts cost $6.79, $11.08, and $8.95. What is the total cost?
$6.79 + $11.08 + $8.95 = $26.82.
A trip is 250 miles. A driver has gone 182 miles. What does the missing part of the bar mean, and what is it?
It means the miles left. 250 - 182 = 68 miles.
Use compensation to solve 11.4 - 6.8.
11.4 - 7.0 = 4.4. Add back 0.2. The answer is 4.6.
Estimate 53.8 - 27.5 using compatible whole numbers.
54 - 28 = about 26.
Is 0.52 + 0.61 less than or greater than 1? Explain.
Greater than 1. The sum is 1.13, so the pieces make one complete whole and 0.13 more.
A rectangle is 75.23 meters wide and 105.55 meters long. Which expression finds its perimeter?
75.23 + 75.23 + 105.55 + 105.55 = 361.56 meters.
Two tacos cost $2.35 each. Chips cost $1.85 and lemonade costs $1.60. A $5 gift card is used, then a $20 bill. Find the change.
2.35 + 2.35 + 1.85 + 1.60 = 8.15. Then 8.15 - 5 = 3.15. Finally, 20 - 3.15 = $16.85.
Use compensation to solve 35.7 - 14.8.
35.7 - 15 = 20.7. Add back 0.2. The answer is 20.9.
Estimate 29.4 + 53.2 by rounding to whole numbers.
29 + 53 = about 82.
Two block sets represent 0.44 and 0.87. What is their sum?
0.44 + 0.87 = 1.31. Regroup to make 1 whole, 3 tenths, and 1 hundredth.
Find the difference: 22.05 - 19.64.
2.41. Line up the decimal points and regroup.
Monica buys items for $59.95, $24.95, $9.75, and $13.50. She uses a $5.50 coupon. What does she pay?
59.95 + 24.95 + 9.75 + 13.50 = 108.15. Then 108.15 - 5.50 = $102.65.