Strategies for adding 2- and 3-digit numbers
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- Which number line shows 361 + 544? Let's see, in all of them we're starting at 361, so now let's add 544. This one starts with adding 400, and then 50, and then 4, it's adding 454, not 544. Now this one adds 500, then 40, and then 4, so this is adding 544, to 361, we see that it is 905. This makes sense, you add 500, you get 861, you add 40, you get to 901, and then add the 4, you get 905, so definitely go with that choice. This one over here, instead of add 500, you add 50, which doesn't make sense because we're clearly adding 5 hundreds, 4 tens, or 40, and then 4 ones, let's do some more examples. Which expression can be solved with the following number line? Let's see, we're starting at 718, and then we're adding 200, and then we're adding 40 to get to 958. This is really 718 + 240, which is this choice right over there. This is kind of, strangely fun. Which number line shows 585 - 368? Let's see, these second two number lines, both start at 585, we're starting our computation at 585, this one's starting at 558, so it swapped the numbers, so this one doesn't make sense, but let's look at these two choices. We want to subtract 3 hundreds, 6 tens, and 8 ones. In both of these, we subtract 3 hundreds, and then we want to subtract 6 tens, 6 tens, and then 8 ones, that's this one over here. This one swaps the... instead of having 6 tens and 8 ones, it has 8 tens and 6 ones, so it swapped... this is subtracting 386, which isn't the original problem, so definitely like this third choice. Let's do one more example. Which expression has been solved with the following number line? We're starting at 935, and then we're going to subtract 400, and then we're going to subtract 20. This is 935 - 420. 935 - 420, that choice right over there.