Subtract within 20 using a number line
Sal subtracts 17 - 9 using a number line. Created by Sal Khan.
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- How can 17 minus 9 also be written as 8=17 minus 9?(1 vote)
- 8 is the difference of 17 - 9, so 17-9=8 is also 8=17-9 or 9=17-8(1 vote)
- [Instructor] We are told subtract, and they want us to subtract nine from 17 or to figure out what is equivalent to 17 minus nine. And they give us a hint. They say, "Use the number line to solve 17 minus nine." So what's going on over here? So on this number line, we can see 17 right over here, and it looks like they took nine steps to the left. Which makes sense, because we are subtracting nine. We are taking nine away. So pause this video and using this hint, think about what 17 minus nine is equal to. All right, well, if we start at 17, as we do right over here, and if we were to take nine steps back, where do we end up? Well, let's see. One step, two steps, three steps, four steps, five steps, six steps, seven steps, eight steps, nine steps. Looks like it gets us to, well, it took us seven steps back to get to 10, and it looks like we took two more steps. So this would be nine right over here, and then this is eight. So it looks like 17 minus nine is equal to eight. Or another way we could write that is eight is equal to 17 minus nine, and we're done.