– [Voiceover] Let’s say you’re

in the babysitting business and you like to keep a log

of whom you are babysitting. So in the last month

you babysat six children and you wrote the ages of

all six children in your log. But then when you go back

to your log you notice that some blue ink spilled

over one of the ages and you forgot how old that child is. And at first you’re really worried, your whole system of

keeping records seems to… you know, you’ve lost information. But then you remember that every time you wrote down a new age that month, you recalculated the mean. And so you have the

mean here of being four, the mean age is four for the six children. So given that, given

that you know the mean, and that you know five

out of six of the ages, can you figure out what the sixth age is? And I encourage you to pause the video and try to figure it out on your own. So assuming you’ve had a shot at it. So let’s just call this missing age, let’s call that question mark. So let’s just think about

how do we calculate, how would we calculate a mean if we knew what question mark is? Well, we would take the total. We would take the total of ages, of ages, we would then divide that by the number of children. We then divide that by the number of ages that we had, and then that would be equal to, that would be equal to the mean. Or another way to think about it, if you multiply both sides

times the number of ages, the number of ages on that side and the number of ages on that side, then this is gonna cancel with that, and we’re gonna be left with the total The total is going to be equal to, is going to be equal to the mean times the number of ages. Mean times, and I’ll just

write times the number, times number of data points, or number of ages. So maybe we can use this information, ’cause we’re just going to have this missing question mark here and we know the mean and we

know the number of ages. So we just have to solve

for the question mark. So let’s do that. So let’s go back to the beginning here, just so this makes

sense with some numbers. The total of ages, that’s going to be five plus two plus question mark, plus question mark, plus two, this two, plus two plus four plus eight. We’re gonna divide by the number of ages. We’re gonna divide it

by the number of ages. Well we have six ages here. One, two, three, four, five, six. Six ages. And that’s going to be equal to the mean. This is going to be equal to the mean. The mean here is four. So let’s see, and this is just

how you calculate the mean. So let’s see if we can simplify this. So five plus two is seven. Let me do this, that’s the wrong color. Five plus two, five plus two is seven. Two plus four is six plus eight is 14. 14. And then seven plus 14 is 21. So we’re left with 21 plus question mark over six is equal to four. Now we can do what we did

when we just wrote it all out. We can multiply both sides

times the number of ages, the number of data points we have. So we can multiply both sides times six. We can multiply both sides, both sides times six. So six on that side, six on this side. Six in the numerator, six in

the denominator, those cancel. So all we’re left is, on the left-hand side we’re

left with 21 plus question mark. Alright. All of these other green numbers, those just simplified, five plus two plus two

plus four plus eight is 21 and we still have the question mark. So we get 21 plus question mark, I want to do that green color, 21 plus this question mark, the thing that we’re trying to solve for. The missing number is

going to be equal to, is going to be equal to four times six. Well what’s four times six? That’s 24. And so what’s the question mark? 21 plus what is equal to 24? And we could, of course, you might just say, well

it’s gonna be three. Or, if you want to, you could say well, question mark is going to be, question mark is going to be equal to, is going to be equal to 24 minus 21. Which is, of course, three. Which of course, so let

me just write this down, so the question mark is equal to three. So the missing age, you

were able to figure it out based on the information you had, because you had the mean, you were able to figure out

that behind this blotch, that behind this blotch you had a three. It’s exciting.

First comment.

Perfect

What about two missing ages?

There are 6 numbers and the mean is 4. Adding all the numbers there are shown is 21, and you would divide the number by how many numbers there are, that's 6, so after it would be 4×6 which is 24, so the missing number is 3.

Thank You Sal

A=k*r^2

A=100

k=4

What is the value of (r)

Can anyone show the working out please thank u.

Thanks Sal!! worked out perfectly!!

lol 21

Why would someone put a thumbs down? He is trying to teach you math.

what number is missing 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Thank you for the help!

Oh M G this is the most complicated expectation ever

what program do you use for your drawings?

thx bro

great video. I enjoyed it very much.

I'm 26 years old. I'm amazed this is for 6th graders 🙂

Awesome, really made very easy, may i know what program younuse to teach? Since it looks very user friendly

Thank you so much! I'm having a pre-algebra equations quiz soon and this type of question is going to be on there but my teacher went over this once. So I was really confused.

the case of the missing number is solved..

you are the best!!! I missed one class and I watched your video and got my homework done!! thank you!!!

i knew it

so it's like working backwards right?

Thanks, this helped so much. I was totally paying attention in class today XD.

how do we calculate the memes

What about if you don't know the mean

this helped but the method is way to long.

thx helped

aGAIN kHAN ACADEMY GETS ME OUT OF ANOTHER GRADE 8 MATH CLASS jam… whoo hoo nice bit of internet searching on my part as well… yipeee, great way to end the year!!! thank you!!!! see you in september

is there any shorter method?

this really helped me, thx

thk you bro 🙂 you explaination is so much clearer

Thank you so much, this was extremely helpful 🙂

Well solved

this seriously helped

m e m e

Thx this was very helpful

If you want a simpler way to solve or comprehend problems like this, I'll show you a trick that I developed randomly while I was solving my homework. I will use the same example used by Sal in this video.

First, do the mean times # of ages in this case. 4 times 6 = 24

Then, add all of the # of ages that you already have. 5+2+2+4+8=21

Finally, subtract your mean from the total # of ages you have. 24-21=3

Both methods that Sal and I used got the same result but in a similar yet different way.

Thanks for tha help

OMG I didn’t get it at the start and now you have helped me complete my homework thanks

Thx

You can also-

1. Add all values from the data (table).

2. Multiply the mean/average by the number of how many values there are.

3. Subtract the product from the sum, or vise versa.

Thank you so much, I've been having some trouble when it comes to those questions because when I do my practice booklet, the answer page doesn't show the process of getting to the answer. This video helped me so much, thanks Khan.

Khan, will you help me by making a video on finding 2 missing frequencies when 2 mean of distribution is given? Please…

Why are you even popular??!! Khan academy just makes me feel dumb.. but everytime i go to other channels on youtube like the organic chemistry tutor, i understand everything so easily… :/

Thx sal you really helped and don’t let the diss likes get to you

Thank so much

Thanks

What if you only have one value and need to calculate more?

You saved my life thank you

Can you do a series on the ged test like you do the SAT test In your app?

Also can you put this as one of your practice questions in the app so we can practice doing it?

it really helped me

thanks a lot

Thanks

what is the missing term in the given series 5,7,28,45,____?

Not so helpful, sir please amend it

Lol

Ha EASY

why is there no sound

for people who can hear the sound just turn the subtittles on

Simpler method

Add the data or Ages: 5+2+2+4+8 = 21

Multiply the mean to the

AMOUNTof the data or ages: 4 x 6 = 24Subtract The SUM of the Ages to the PRODUCT of the mean and amount of ages: 24 – 21 = 3

The missing data or age is 3

Or you could just call the parents and ask. XD thanks, though, I have math test today, and didn't know how to do this TwT

I'm having one issue, we are asked to find the missing numbers in a group data but the the total frequency number is not given, please I need help. Thank

First video of Khan Academy!!!

What do you do when there is more than 1 missing number?

What if the mean is missing too?