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Here are the rules for multiplying decimal numbers:
Example
37.7 x 2.8 = ? >
37.7 ( 1 decimal place )
x 2.8 ( 1 decimal place )
3016
+754
105.56 ( 2 decimal places, move point 2 places left )
Hint: Use estimating to help you check the placement of the decimal point. You could round 37.7 to 40 and 2.8 to 3. It's easy to multiply 3 x 40 so you know your answer should be close to 120.
Here's a "mental math" shortcut: When multiplying a number by a multiple of ten, just move the decimal point one space to the right for every zero.
10 x 0.6284
= 6.284 (1 zero, 1 space right)
100 x 0.6284 = 62.84 (2 zeroes, 2 spaces right)
1000 x 0.6284 = 628.4 (3 zeroes, 3 spaces right)
10,000 x 0.6284 = 6284 (4 zeroes, 4 spaces right)
100,000 x 0.6284 = 62,840 (5 zeroes, 5 spaces right)
Example
Line up the numbers on the right, multiply each digit in the top number by the each digit in the bottom number (like whole numbers), add the products, and mark off decimal places equal to the sum of the decimal places in the numbers being multiplied.
9.683
( 3 decimal places)
x 6.1 ( 1 decimal
place)
9683 (1 x 9683)
+ 58098 (6 x 96830)
59.0663 (3 + 1 = 4 decimal places)
First, estimate your answer to the tens place. Then make an exact calculation, using the calculator.
Estimate:
24.5 > 20 15.2 > 20 20 x 20 = 400 miles
Exact calculation:
24.5 x 15.2 = 372.40
You can drop the trailing zero in a decimal number. For example, 372.40 is the same as 372.4. This is because the ".40" part of the number is the same as ".4". This is the same as saying that 40/100 is the same as 4/10.
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