Find a Missing Value From the Mean

Find the Missing Value From the Mean

When the mean is known and one value is missing, use the mean definition:

mean = total sum ÷ number of values

1) Write the total sum

If the mean and the number of values are given, multiply them:

total sum = mean × number of values

This gives the sum of all data values together.

2) Add the known values

Add the values that are already given in the problem.

3) Subtract to find the missing value

Take the total sum and subtract the sum of the known values:

missing value = total sum − known sum

If there is more than one missing value, combine them first and solve for the total of the missing parts.

4) Check your result

Replace the missing value in the original list, then recompute the mean. If the mean matches the one given, the answer is correct.

Example method

• Mean = 8
• Number of values = 5
• Total sum = 8 × 5 = 40
• Known values sum to 31
• Missing value = 40 − 31 = 9

Important tip

Always simplify the final answer and make sure it fits the context of the data.