Solving two-step word problems involving adding and subtracting decimals
About this video
Ali travels 51.84 km to work by car (6.07 km), walking (1.3 km) and train. The video finds the train distance by subtracting the walk and car distances from the total, lining up place values and regrouping when needed. The answer is 44.47 km.
You'll be able to
- write a subtraction to find the missing part of a total in a word problem.
- subtract decimals by lining up place values.
- regroup a tenth as 10 hundredths when subtracting decimals.
Key words
- place value
- The value of a digit based on its position, such as tenths or hundredths.
- regrouping
- Trading one unit of a place for ten of the next smaller place, like 1 tenth for 10 hundredths.
- hundredths
- The second place to the right of the decimal point; 100 hundredths make 1.
Transcript
- [Teacher] We are told it takes Ali a total of 51.84 kilometers to get to work. She travels 6.07 kilometers by car, 1.3 kilometers by walking, and the rest by train. How many kilometers is Ali's train ride? Pause the video. Have a go at it before we do it together.
Okay, so if we were to add the train ride plus the walk, plus the car, it should total up to 51.84 kilometers. Now, we know what the walk is and what the car ride is. We just need to figure out the train ride. So, another way to think about it is the train ride should be equal to the total, 51.84, minus the amount that we walk.
Minus the amount that we walk. And now, what is that? That is 1.3 kilometers. Minus 1.3. And then also, minus the amount that we go by car. The amount that we go by car is 6.07 kilometers. So, we just need to figure out what this is. So, let's do that, and let's do this.
Let's first subtract the 1.3 and then, let's subtract the 6.07. So if I first try to calculate this part right over here, I have 51.84, minus, make sure to line up the place value, 1.3. All right, 4 hundredths minus nothing, minus 0 hundredths, is 4 hundredths.
8 hundredths. Make sure I pronounce it right. 8 tenths minus 3 tenths is 5 tenths, and then 1 minus 1 is 0 and then 5 minus 0 is 5. So, this right over here is 50.54 kilometers, .54. Now, let's subtract 6.07 from that. And I could actually just do that right over here, so let's say minus 6.07.
All right, so we go to the hundredths place. We can't subtract 7 from 4. A little bit of regrouping. Let's take a tenth, so now we only have 4 tenths here. A tenth is the equivalent of 10 hundredths, so now we have 14 hundredths. And so, we could subtract 14 minus 7 is 7, 4 minus 0 is 4.
We have our decimal here. And we could do regrouping again or you might just recognize that 50 minus 6 is going to be 44. If we wanted to regroup, we could say, "Hey, let's take a 10 here," and then that's 10 ones. 10 minus 6 is 4, and then you bring down those 4 tens.
But either way, the amount that Ali travels by train is 44.47 kilometers.
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