Adding and Subtracting Decimals

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by 5th Grade Northridge1 view·Updated Oct 18, 2020
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how to add and subtract decimals in real world problems.

Audio TranscriptAuto-generated

  1. Hey, guys.
  2. And welcome back to another math lesson.
  3. Today's lesson is, um, adding and subtracting decimal.
  4. I hope you have your notebooks and pencils ready, and
  5. you need to write this at the very top of
  6. the next empty page, adding and subtracting decimals as usual.
  7. We have a goal.
  8. Today's goal is I can add and subtract decimals.
  9. Very simple.
  10. Of course, if you read the title, you know the
  11. title, then you would know the goal is very simple.
  12. Very easy.
  13. Um, I can add and subtract decimals.
  14. Like always.
  15. There are rules to math.
  16. There are two rules for this lesson.
  17. Rule number one is line up.
  18. The decimals line up the decimals.
  19. So when we line up the decimals, it means that
  20. all the decimals of all the numbers line up even
  21. the answer.
  22. So if we had numbers in here, say it Waas
  23. two and 7/10 plus three and 16th, then when we
  24. added them, we would go three carry little one and
  25. six, so the decimals would line up all the way
  26. through Rule number one.
  27. Line up the decimals rule number to add or subtract
  28. from left to right.
  29. That means If there's a problem like this one, I
  30. would begin working it from the left side of the
  31. problem from the left to the right.
  32. So I would do five plus seven first, which is
  33. 12 minus three plus nine.
  34. You should remember this from orders of operations 12 minus
  35. three, which is nine.
  36. Last night and then I'm plus nine, which equals 18
  37. and that would give us our problem.
  38. Now, of course, when you're doing, um, decimals, your numbers
  39. will be a little bit bigger.
  40. You will actually have decimals to add or subtract, but
  41. that doesn't matter.
  42. You would still start on the left side and go
  43. through to the right so that second rule is add
  44. or subtract from left to write.
  45. Now, I'm going to stop right here and let you
  46. answer some addition and subtraction problems using decimals.
  47. All right, Now I have something that I need you
  48. to remember.
  49. There's one more thing.
  50. If you have a word problem, I need you to
  51. think What do I know?
  52. What are all the things that they have given me
  53. in the word problem?
  54. What are the things that I know, and then what
  55. do I need to know to solve the problem.
  56. What am I trying to find out?
  57. You have to be able to ask yourself these questions.
  58. What do I know?
  59. What do I need to know?
  60. To solve the problem.
  61. For example, how can I get the answer?
  62. What do I need to do?
  63. Add, Subtract, multiply, Divide?
  64. How can I get there?
  65. What do I know?
  66. What do I need to know?
  67. How can I get there or get the answer?
  68. Let's have an example.
  69. You have $35.50.
  70. You buy a movie ticket for $15.
  71. 89 cents and music for $12.50.
  72. How much do you have left?
  73. Well, what you know is you have $35.50.
  74. You also know that you bought a movie ticket for
  75. $15.89. So you started out with $35.50.
  76. Then you took away $15.89 because you bought a movie
  77. ticket and music for $12.50.
  78. So after you bought your movie ticket, you spent $12.50
  79. to get moved to get music and what you want
  80. to know.
  81. It's how much you have left.
  82. So now we can answer this problem.
  83. What do you know?
  84. You know, I have $35 and 50 cents.
  85. I know that I subtract it or took away $15.59
  86. because I went to the movies.
  87. I also know that I took away $12.50 because I
  88. went and got some music.
  89. And what I want to know or need to know
  90. is how much is left.
  91. Well, that's pretty easy.
  92. All we have to do now is take away or
  93. subtract the numbers that we spent.
  94. This is how you break down a word problem.
  95. What do I know?
  96. What do I need to know?
  97. How can I figure it out?
  98. So now you can answer the problem by help.
  99. $35.50 and I have to subtract $15.89.
  100. I have to do that First 10 or that's one
  101. 14 or that's six one to nine.
  102. Remember my decimals line up all the way down and
  103. one, but that's not the end.
  104. That's how much I have after the movie I also
  105. went and bought music on the music Waas.
  106. The music was $12 at 50 cents.
  107. So of course, all I have to do now is
  108. subtract how much I got from the music to get
  109. the right answer.
  110. I'm going to do that over here on my camera.
  111. Let's give you guys can see.
  112. So if I had $19 and 61 cents and I
  113. had to subtract it from $12.50 because that's how much
  114. I spent at the movies.
  115. Now I can subject again 11 seven.
  116. And that is how much I have left.
  117. Alright, guys.
  118. So that answer would be $7.11.
  119. Now, I want you to go and do a few
  120. word problems.
  121. Practice them and I'll see you next time.