How to Find the Missing Number of the Given Mean

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The mean of a set of numbers is the average of those numbers. You can find the mean by adding the set of numbers and dividing by how many numbers are given. If you are given the mean and asked to find a missing number from the set, use a simple equation.

  1. Given the Numbers

  2. Add up the numbers you know. The problem states a mean of 58 with this set of numbers: 43, 57, 63, 52 and ​x​. Assign the missing number a value of “​x​.” So add 43, 57, 63 and 52 to get 215.

  3. Setting Up the Equation

  4. Set up your equation by adding 215 plus “​x​” (the missing number), divided by 5, the number of values given. Set that side of the equation equal to the mean, 58. So, your equation would look like this:

  5. Isolating "x"

  6. Multiply each side by 5 since our goal is to get “​x​” by itself. This process cancels the 5 on the left side of the equation and gives you 290 on the right side (58 × 5). Now, your equation should look like this:

    215+x = 290
  7. Solving for "x"

  8. Subtract 215 from each side as you continue to work to get “​x​” alone. This cancels out the 215 on the left side of the equation and gives you 75 on the right side. Now, your equation should show that ​x​ = 75. Therefore, the missing number is 75.

  9. Check the Answer

  10. Check the missing number by adding all the numbers together and dividing by 5.

    \frac{43+57+63+52+75}{5}=\frac{290}{5}= 58

    Which is the given mean.



About the Author

Tamara Runzel has been writing parenting, family and relationship articles since 2008. Runzel started in television news, followed by education before deciding to be a stay at home mom. She is now a mom of three and home schools her two oldest children. Runzel holds a Bachelor of Arts in communication from University of the Pacific.