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## 4th grade

### Course: 4th grade > Unit 4

Lesson 1: Multiply by 10s# Multiplying 10s

Lindsay multiplies multiples of 10 by other multiples of 10, such as 30x50.

## Want to join the conversation?

- why do you have to have a pattern?(31 votes)
- Everything in math tends to have a pattern, math can be confusing at times so it is very helpful to go back to a pattern or rule to help focus and solve the question.(6 votes)

- An
**easier**way to do it is to just to do for example in*50x30*do**5x3**which is**15**then make it into a**thousand**(17 votes)- Good thought! In your 50 x 30 you will note that you have two zeros on those numbers. If you then multiply 5 x 3 to get 15 you just need to add those two zeros back on to get 1500.

There are lots and lots of "tricks" like that you will discover as you go along. Then math becomes kind of fun and easy.(26 votes)

- also i have another question if i were to multiply 10 would that mean it would just be counting up by tens so like if i did 2 times 10 it would be 20 cause it's in the tens place?(10 votes)
- Yep! you're correct :) Multiplications Tables is also adding numbers the same numbers to each other so... 10+10+10 or 5+5+5, find a Times Table Grid on Google that you like and use it to help you learn your Times Tables.

A tip to help you learn them is 2, 4, 6 and 8 Times Tables have a 5 Digit repeating number pattern from the first 5 numbers then repeats in the right side numbers. 1 at a time learn them and say the patterns to yourself, then practise them by writing the patterns then fill them in the other numbers. Eventually you won't have to use the pattern for 2 times table that much once you feel you've learned them well but you'll still need it for the rest.

2 Times Table goes... 2 4 6 8 0

4 Times Table goes... 4 8 2 6 0

6 Times Table goes... 6 2 8 4 0

8 Times Table goes... 8 6 4 2 0

3 and 7 times tables is tough to learn because it has a 10 digit number pattern but 3 times table because it's a small number it's easy to add 3+3+3 in your head but with 7 times table use the other number in your multiplication sum to help you work it out instead because it doesn't matter which way you multiply or even add two or more numbers together, it will always give you the same answer :) But remember that 7x7 is 49.(20 votes)

- wait so if i were to do 40 times 70 it would be 1100?(0 votes)
- no 7 x 4 = 28 and then add zero's it would be 2800(35 votes)

- 900+800=1700-700=1000+400+170=i dont know(12 votes)
- the answer is 1,570(6 votes)

- Do you time us?🤔(10 votes)
- Were are the questions?(9 votes)
- Idk I'm asking the same thing person from 2 years ago.(2 votes)

- Hi I have a lot of fun with you guys I hope you’re having fun to(9 votes)
- not to be rude but you could just do 4x7=28

and then just ad the two zeros(7 votes)- Yes but this carefully shows
*why*you can do that(1 vote)

- why is khan academy so boring!(4 votes)

## Video transcript

- [Voiceover] Let's multiply 40 times 70. So 40 times, we have the number 70. So we could actually list that out, the number 70, 40 different
times and add it up. But that's clearly a lot
of computations to do. And there's gotta be a faster way. So another way is to
stick with multiplication, but see if we can break these numbers up, this 40 and this 70, decompose them, break them up in some way to get numbers that might be a little
easier to multiply with. For me, multiplying by
10 is the easiest number, because I know the pattern to add a zero. So, I'm gonna break up 40
and say, instead of 40, four times 10. Four times 10 and 40 are equivalent. They're the same thing,
so I can replace the 40 with a four times 10. And then for my 70, same thing. I can break this up and
write seven times 10. Seven times 10. So these two expressions, 40 times 70 and four times 10 times seven times 10, are equal; they're equivalent. So they'll have the same solution. But for me, this one down here is simpler to work out because of these times 10s. So I'll solve this one, knowing that I'll get the same
solution as I would have for this top expression. So what we can do is we
can re-order these numbers in a different order to, again, continue making this question
easier for us to solve. Because in multiplication,
the order doesn't matter. If we have five times two, for example, that would be the same as two times five. They're both 10. Five twos or two fives,
either way, it's 10. So we can change the order of the numbers without changing the answer. So again, we're going to change
our expression a little bit, but what we're not going
to change is the solution. So I'm gonna put my
one-digit numbers first. Four times seven. And then, I'll put the
two-digit numbers, the 10s, times 10 and the other times 10. So we have all the same factors, all the same numbers, in
both of these expressions. They've just been re-ordered. And now, I'll solve going across. Four times seven is 28. And now we have 28 times 10, and times another 10. Well, the pattern for
times 10 that we know is when we multiply a whole
number like 28 times 10, we will add a zero to the end. One zero for that zero in 10, because 28 times 10 is 28 10s, 28 10s, or 280. And that multiplied 28 times 10, and then if we multiply by this other 10, well we have to add another zero. Multiplying by 10 adds a zero, so if we multiply by two
10s, we add two zeros. So, 28 times 10 times 10, is 2,800. Which means that this
original expression we had, 40 times 70, also has a solution of 2,800, or two thousand, eight hundred. Let's try another example where we're multiplying 10s like this. Let's try, let's do
something like, let's say, maybe 90 times, how about, 30. 90 times 30. So the first thing I'm gonna
do is break up these numbers so that I have 10s, because again, for me 10s are easier to multiply
than numbers like 90 and 30. So for 90, I'll write nine times 10, and for 30 I'll write three times 10. The expressions are equivalent. We've just written it in another way. And now, I'll re-order these numbers to put the one-digit numbers first. So, nine times three, and then I'll put the
10s: times 10, times 10. Because we need to have
all the same numbers, even if we change the order. So we have the nine, three, the first 10 and the second 10. And now, finally, we multiply. Nine times three is 27. 27 times 10 will be 27 10s, or 27 with a zero on the end. And 270 times 10 will be 270 10s, or 270 with a zero on the end. So going back to the original question, 90 times 30 is equal to 2,700. Or two thousand, seven hundred.