Video: Adding Mass Quantities

If a man who weighs 67 kg is riding a donkey that weighs 171 kg, what is the total of their weights?

03:28

Video Transcript

If a man who weighs 67 kilograms is riding a donkey that weighs 171 kilograms, what is the total of their weights?

The word total in the question is a hint to us that what we need to do to find the answer is to add some numbers together. And in this particular problem, we’re adding together quantities of mass. And we’re given two measurements of mass. We’re told that a man who weighs 67 kilograms ⁠— so our first number is 67 ⁠— is riding a donkey that weighs 171 kilograms. And so to find the total, we need to add 67 and 171. Now, we know that when we add any numbers together, we can add them in any order. And the total will still be the same. So let’s start by swapping our two numbers around so that we’re starting with the larger number. And then we don’t have to add on so much. So we can think of the calculation as 171 plus 67.⁠

Now, because we’re adding a two-digit number, it makes sense to add the ones digit first and then the tens. And so that each digit has the correct value, let’s set out the numbers vertically in a column addition, 171 plus 67. What we’ll do at the same time is to model the calculation using base ten blocks. 171 is one hundred, seven tens, and one one. And 67 can be represented by six tens and seven ones. We can start by adding the ones digits together. What’s the total of one and seven? One plus seven equals eight. Next, we need to add our tens digits. What are seven tens plus six tens? Now, we know seven plus six equals 13. But we can’t write two digits in the tens column. We can’t write 13 tens. But we’ve got enough tens here to exchange some of them for one hundred.

We know 10 tens are equal to 100. So we can take 10 of our tens blocks and exchange them for one hundred. So now we have a three in the tens column and one hundred underneath in the hundreds column that we’ve exchanged. Finally, if we add our hundreds column, we only have the one hundred in the top number and then also the one hundred that we’ve exchanged underneath. This makes two hundreds altogether. 171 plus 67 equals 238. And if we look at our base ten blocks, we can see two hundreds, three tens, and eight ones, 238. If a man who weighs 67 kilograms is riding a donkey that weighs 171 kilograms, we can find the total of their weights by using column edition. And that total weight is 238 kilograms.

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