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CCSS Math: 3.OA.D.8

A theater has a total of 63
seats in a rectangular grid. Here they are, right over here. It currently has 9 rows, each
with an equal number of seats. We see that-- 1, 2, 3,
4, 5, 6, 7, 8, 9 rows. How many total
seats will it have if it adds 2 seats per row? So what we want to figure out
is the total number of seats. So let's say that
the total is going to be equal to the
number of seats that the theater already has. And they tell us that they have
63 seats in a rectangular grid. So it's going to be 63
seats plus the new seats, plus however many it adds. So plus the new seats. Well, what are the
new seats going to be? Well, the new seats are
going to be-- I have 9 rows, and I'm adding 2 seats per row. So the new seats are going to
be 9 rows times 2 seats per row. So we could say
that the total is going to be equal to
63 plus 9 times 2. And we 'll talk more
about order of operations. But clearly, in
this situation, we want to multiply 9
times 2 to figure out the total new number of seats. And then we want
to add that to 63. And if we just
write it like this, multiplication takes-- you
will always do multiplication. And when you just see
it in a row like this, you'll do the multiplication
before you do the addition. So this is literally
saying, hey, multiply 9 times
2 first, then add to 63, which is exactly
what we care about. And what is 9 times 2? Well, let's add those
2 seats per row. So that's 2, 4, 6, 8,
10, 12, 14, 16, and 18. So 2 times 9. 2, 4, 6, 8, 10, 12,
14, 16, 18 is 18. So we're adding 18 new seats. So let me write this down. I covered up my old total. So total is equal to 63 plus 18. I had 63 seats. I'm now adding 18 seats. So total-- my total
number of seats-- is now going to be 63 plus 18. Let's see, 8 plus 3 is 11. 60 plus 10 is 70. 70 plus 11 is going to be--
I have 80-- I have 81 seats. I now have 9 rows with 9 seats
each, which gets me 81 total.