Section outline

  • lesson content

    Marlo is having a party. He needs to buy the party snacks and drinks from different stores. Marlo spends $21.60 at the first store and $19.40 at the second store. He's wondering if he can afford to go to a third store for more snacks. He doesn't have a calculator but wants to quickly figure out how much he's spent so far so. How can mental math help calculate how much Marlo has already spent on snacks?
    ::Marlo正在开派对。 他需要从不同的商店购买聚会的零食和饮料。 Marlo在第一商店花21.60美元,在第二商店花19.40美元。 他想知道他能否买得起去第三商店买更多的零食。 他没有计算器,但他想很快弄清楚他目前花多少钱。 心理数学如何帮助计算Marlo已经花在零食上多少钱?

    In this concept, you will learn how to use mental math to add and subtract decimals.

    ::在此概念中,您将学会如何使用心理数学来增减小数。

    Sums and Differences Using Mental Math 
    ::使用心理数学的总数和差异

    Sometimes, you don’t need to go through all of the work of lining up decimal points and filling in the zeros. Sometimes mental math is a fast way to find the  sum  of two decimals.
    ::有时,你不需要去研究所有将小数点排在小数点和填充零的工作。 有时,心理数学是找到两个小数点之和的快速方法。

    When you have a decimal where the decimal parts can easily add up to be one whole, you can use mental math to figure out the sum. Think about this. If you had 0.30 + 0.70, you know that 3 + 7 is 10, therefore you know that 0.30 + 0.70 is 1.00.
    ::当您有一个小数点的小数点, 小数点部分可以很容易地合并为一个整体时, 您可以使用心理数学来计算总和。 仔细想想。 如果您有0. 30 + 0. 70, 你知道 3 + 7 是 10, 所以您知道 0. 30 + 0. 70 是 1. 00 。

    Let’s look at an example.
    ::让我们举个例子。

    5.30 + 6.70

    Start by looking at the decimals. 0.30 + 0.70 is 1, then combine the  whole numbers  and add the total of the decimals to get an answer:
    ::从小数点0.30 + 0.70为 1开始,然后将整数合并,并加上小数点的总和以获得答案:

    0.30 + 0.70 = 1 Here is the decimal part 5 + 6 = 11 Here is the whole-number part 11 + 1 = 12 Here is the total

    The answer is 12.
    ::答案是12岁

    Sometimes you can also use mental math to figure out the  difference  of two decimals.
    ::有时您也可以使用心理数学来计算小数点后两个小数点的差数。

    Look for which decimals add up to a whole and go from there.
    ::查看小数点加起来等于一个整体然后从那里开始。

    Let's look at an example.
    ::让我们举个例子。

    25.00 − 22.50

    First, notice that 0.50 can easily be paired with 0.50 to equal one whole.
    ::首先,请注意0.50可以很容易地配上0.50到等于一个整体的0.50。

    Next,  round  22.50 to the nearest whole number and subtract that value from 25.
    ::下一轮,22.50轮到最近的整数,再从25次减去该数值。

    25 − 23 = 2

    Then, combine the decimal and the whole number.
    ::然后,将小数点和整个数字结合起来。

    2 + 0.50 = 2.50
     

    The answer is 25.00 - 22.50 = 2.50
    ::答案是25.00 -22.50=2.50

    Examples
    ::实例

    Example 1
    ::例1

    Earlier, you were given a problem about Marlo shopping for party snacks.
    ::之前,有人给你一个问题 关于马洛购物 为聚会零食。

    He spends $21.60 at one store and $19.40 at a second store. Marlo wants to quickly combine these totals. Can you use use mental math to find the sum of $21.60 and $19.40?
    ::Marlo想快速合并这些总数。你能用心理数学来找到21.60美元和19.40美元吗?

    First, add the decimals.
    ::首先,加上小数。

    0.60 + 0.40 = 1

    Next, add the whole numbers.
    ::接下来,加上全部数字。

    21 + 19 = 40

    Then, combine the two sums.
    ::然后,将这两笔金额结合起来。

    40 + 1 = 41

    The answer is that $21.60 + $19.40 = $41.00. Marlo spent a combined total of $41.00 at the two stores.
    ::答案是21.60美元+19.40美元=41.00美元,Marlo在两家商店共花了41.00美元。

    Example 2
    ::例2

    Kiley went shopping and spent $5.60. She gave the clerk a ten dollar bill. What was her change?
    ::凯利去购物,花了5.60美元 她给了办事员10美元

    First, what decimal added to 0.60 makes it a whole number?
    ::首先,什么小数点加到0.60, 使它成为一个完整的数字?

    0.40

    Next, round 5.60 to the nearest whole number and subtract that from 10
    ::下一轮,5.60轮至最近的整数整数,减去10轮整数,再减去10轮整数

    10 − 6 = 4

    Then, combine the decimal and the whole number.
    ::然后,将小数点和整个数字结合起来。

    4 + 0.40 = 4.40

    The answer is 10 - 5.60 = 4.40.
    ::答案是10 - 5.60=4.40。

    Example 3
    ::例3

    Add using mental math.
    ::加上心理数学

    33.50 + 5.50

    First, look at the decimals.
    ::首先,看看小数点

    The decimals easily add up to one whole.
    ::小数点很容易将一个整数相加。

    Next, combine the decimals and the whole numbers separately.
    ::下一步,将小数和整数分开合并。

    0.50 + 0.50 = 1 33 + 5 = 38

    Then, add the two sums together.
    ::然后,你们将两笔相加。

    38 + 1 = 39

    The answer is 33.50 + 5.50 = 39.
    ::答案是33.50+5.50=39。

    Example 4
    ::例4

    Subtract using mental math.
    ::用心理数学来减法

    10 − 3.75
     

    First, what decimal added to 0.75 makes it a whole number?
    ::首先,在0.75中,小数点加到多少小数点才能算成一个完整的数字?

    0.25

    Next, round 3.75 to the nearest whole number and subtract that from 10
    ::下一轮,第3.75轮至最近的整数整数,减去10次的整数

    10 − 4 = 6

    Then, combine the decimal and the whole number.
    ::然后,将小数点和整个数字结合起来。

    6 + 0.25 = 6.25

    The answer is 10 - 3.75 = 6.25
    ::答案是10 - 3.75=6.25

    Example 5
    ::例5

    Add using mental math.
    ::加上心理数学

    18.25 + 2.25

    First, add the decimals.
    ::首先,加上小数。

    0.25 + 0.25 = 0.50

    Next, add the whole numbers.
    ::接下来,加上全部数字。

    18 + 2 = 20
     

    Then, combine the two sums.
    ::然后,将这两笔金额结合起来。

    20 + 0.50 = 20.50

    The answer is that 18.25 + 2.25 = 20.50.
    ::答案是18.25+2.25=20.50。

    Review
    ::回顾

    Use mental math to compute each sum or difference.
    ::使用心理数学来计算每个总和或差数。

    1. .50 + 6.25 = _____
    2. 1.75 + 2.25 = _____
    3. 3.50 + 4.50 = _____
    4. 7.25 + 1.25 = _____
    5. 8.75 + 3.25 = _____
    6. 8.50 + 2.50 = _____
    7. 10 + 4.50 = _____
    8. 12 + 3.75 = _____
    9. 15.50 - 5.25 = _____
    10. 20 - 15.50 = _____
    11. 10 - 4.50 = _____
    12. 30 - 15.50 = _____
    13. 40 - 16.40 = _____
    14. 75 - 50.50 = _____
    15. 80 - 40.25 = _____

    Review (Answers) 
    ::回顾(答复)

    Click   to see the answer key or go to the Table of Contents and click on the Answer Key under the 'Other Versions' option.
    ::单击可查看答题键, 或转到目录中, 单击“ 其他版本” 选项下的答题键 。

    Resources
    ::资源