Solving two-step word problems with decimals (adding and subtracting)
About this video
Lin starts with $80 and buys gifts costing $17.50 and $22.95. The video shows two ways to plan the problem and then adds the gift costs to get $40.45 and subtracts that from $80.00, regrouping across places, to find Lin has $39.55 left.
You'll be able to
- plan a two-step money word problem in more than one way.
- add decimals by lining up place values and regrouping.
- subtract decimals from a whole number by writing zeros and regrouping.
Key words
- tenths place
- The first place to the right of the decimal point; 9/10 is 9 tenths.
- hundredths place
- The second place to the right of the decimal point; 5/100 is 5 hundredths.
- regrouping
- Trading one unit of a place for 10 of the next smaller place (or 10 for 1 the other way) so you can add or subtract.
- decimal point
- The dot that separates whole numbers from tenths and hundredths; line these up when adding or subtracting.
Transcript
- [Instructor] We are told that Lin has saved $80. He spent 17.50 on a gift for his brother, and 22.95 on a gift for his mom. How much money does he have left after buying the gifts? Pause this video and work through this on your own before we do this together.
All right, now let's think this through. So we know how much money Lin starts with: $80. So let me just write that down. I'm just gonna assume all of this is in dollars. And then how much does he spend? Well, he spends 17.50 for his brother and he also spends 22.95 on his mother.
So there's really two ways, and maybe more, that you could think about how to calculate how much money he has left. You could just say, okay, he starts with $80, we take away $17.50, and then we take away another $22.95. Another way to think about it is he's starting with $80 and then let's just take away the total amount that he spends.
So what I have here in red, the $17.50 plus the $22.95, this is the total amount that Lin is spending on gifts for his family. So what we need to do is just subtract that from the original $80 that he starts with. So let's work through this together. So let's first add $17.50, plus $22.95.
So I'll start in the hundredths place. Zero plus five is five. Go to the tenths place. 5/10 plus 9/10 is 14/10, which is the same thing as 4/10 and a whole. Let me put my decimal there to make sure we have everything lined up. Now, one plus seven plus two, that's 10 ones, or 10 wholes, which is the same thing as zero ones and a 10.
And now we have one plus one plus two, that's four tens. So this is how much Lin is spending. So this is $40.45. And so now, we just have to subtract that from the original $80. And let me write it out all the way. Lemme use that same shade of blue. Let me write it out all the way to the hundredths place.
And I am going to subtract now the $40.45, the total amount that he spent on gifts. And what do we get? Well, it looks like we're gonna have to do some regrouping. We can't take five from zero, four from zero, and the first place where we could take something from is in our tens place.
So let's take one of them away. So this becomes seven tens and 10 ones. Now, let's take one of those away. That becomes nine ones and 10/10. And let's take one of those away. So that becomes 9/10. And then that one we took away, that extra 1/10, that is 10/100.
And now, we are ready to subtract. 10 minus five, 10/100 minus 5/100, is 5/100. Nine minus four, we're in the tenths. So this is 5/10. Put our decimal there. Nine minus zero is nine. Seven minus four is three. And there we have it. He is left with $39.55. That's what all of this business is equal to.
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