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Lesson 5: Mean and median challenge problems

# Missing value given the mean

When babysitting, you forget one child's age from your records. But, you remember the mean age of the six children is four. Using this mean, you can find the missing age. The total of all ages equals the mean times the number of children. So, you solve for the missing age and find it's three.

## Want to join the conversation?

• what can i do to remember what mode means
• Mo de = Mo st occurring number.
Me a n = A verage
M ed i an = Mi ddle
• Is there a simpler way to figure out this question?
• Think the mean as the zero point,then the data would be transform to [+1,-2,<unknown>-4,-2,+0,+4], when you sum them up excluding <unknown>, it results +1,it means you have to fill a number -1 to balance the data,or you can say <unk>-4=-1,where <unk> is equal to 3
• When asked to stop and think about the missing number, my first thought was to multiply the mean by the number of kids/data points - 4 x 6 giving me 24. Then I subtracted all the known ages... 24-8-4-2-2-5 giving me 3 (the same answer).
This seemed easier than the method in the video, but would this method always work for me?
• Yes, it will work, because it is basically the exact same thing that Sal did in the video. The only difference is that Sal added the known ages together before subtracting them from the total.
24 − 8 − 4 − 2 − 2 − 5 = 24 − (8 + 4 + 2 + 2 + 5)
• And is there a simpler way to do this
thx and I would really appreciate it
• I don't know if this is 100% correct, but this is what I did to simplify what he said and get the answer.

My first step was to add everything together to get 21+?.

Since there are six numbers, and the mean is 4, I did 6*4 to get 24.

Last but not least, I did 24-21 to get the missing number.

This method certainly worked for this problem, but I'm not 100% sure if it works for all problems like this? Hopefully someone can clear this up for me. Thanks for reading and (hopefully) answering this question.
• Yes, it comes from the formula. We add (5+2+2+8+4+x)/6 = 4, so you could multiply both sides by 6 to get 21+x=24. If you changed the number and the mean, you would still multiply these two together and then subtract what you know.
• i paused the video and solved it and got 3 but when u explain it i got so confused and forgot how to do the math. explain it properly
• He solved it algebraically, which is a very solid way of answering this type of problem. However if you haven't started learning algebra then it might be very confusing. If you haven't started algebra than think of it as a scale. We know we have 6 values with a mean of 4. So we get 24 on one side. On the other we add all values up 8 + 5 + 4 + 2 + 2 = 21. Then we just subtract 21 from 24 to find the missing number, so 24 - 21 = 3.
• 22 divided by 6 is 3.something, correct?
• Yes 3.6 repeating or 3 and 1/3
• What does Sal mean when he cancels out the 6?
• All that really means is that 6/6 =1, and anything multiplied by 1 is itself.