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# Substituting variables and simple equation

Video transcript

Mario started the day
with 5 fire flowers. He used some and
now only has 3 left. If u is the number
of flowers he's used, we can write the
following equation. How many fire flowers
did Mario use? So he started with 5. He uses some of them, so we
subtract the number he used, and he ends up with 3. So there's a bunch of
ways that you could do it. One way is you literally could
probably do this in your head. Hey, I took away some
number from 5, and I got 3. And you probably know, hey,
well, if I take 2 away from 5, I'm going to get 3,
so he used 2 flowers. The other way that you
could think about doing it is, well, hey, we got
only four options here. Let me substitute
each of them in here. Maybe I was able to do
this one in my head, but if I had something
more complicated, let me try out these choices. So if you only took zero
away, it would be 5 minus 0. He'd still, of course,
be left with 5. If you took 1 away, then of
course, he'd only have 4 left. If he took 3 away,
he'd have 2 left. But if he took 2 away,
5 minus 2 is indeed 3. And the third way, you'll learn
this as we go into more of the how of algebra,
and also the why, is that you could
systematically solve it. But we'll go into
that in future videos. So he used 2 fire flowers.