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Studying for a test? Prepare with these 8 lessons on Addition and subtraction.

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# Subtracting in your head (no regrouping)

Video transcript

What I want to do in this
video is show you a way of subtracting numbers that is
different than the regrouping technique. And this is closer to what
I actually do in my head. And this might not be
what you see in school, so be careful while
you're looking at this. Some people might
not fully approve of what I'm about to show you. But the idea, the reason
why I'm showing you this is to understand that there's
not just one way to do things. As long as you understand
the underlying principles, what these numbers
represent, and you do things that make
sense, you should be OK. And what's neat
about this technique is that we don't
have to regroup. We're just going to start
with the hundreds place and keep on moving. So, for example,
if I think of 301, I first want to subtract 100. So 300 minus 100 is
going to get me to 200. Now I need to subtract 60. And so I can think about what
is 20-- two, zero-- minus 6. So this is essentially
saying what is 200 minus 60? Well, 20 minus 6 is 14. So I just subtract, and
now I'm left with 14. And so now the problem has
resulted in 141 minus 9. So I really just have to
think about what-- well, or I could do what
141 minus 9 is. Well, 141, it's a little
bit more mental computation than you might be used to. But 141 minus 9 is
going to be 132. So we are left with 132. Let's do this one,
same technique. And I encourage you
to pause the video and try it yourself,
this same technique. Well, 9 minus 2, it's
really 900 minus 200. You're going to
be left with 700. Then 71 minus 8, let's
see, 11 minus 8 is 13. So you're going to
be left with 63. And now we have to
subtract 633 minus 6. 13 minus 6 is 7. So it's going be 627. And once again, try to pause
the video and do it on your own. So I don't have any hundreds to
subtract, so I can immediately go to 72 minus 8, which
is really 720 minus 80. But let's just think
in terms of 72 minus 8. Well, 12 minus 8 is 4, so 72
minus 8 is going to be 64. So now this is the same
thing as 641 minus 8. Well, 41 minus 8
is going to be 33. I have to do a little bit
of mental computation here. So this will result in 633.