The Brainliest Answer!
If you know the mean, multiply it by the number of pieces in the set.  For example, if you know the mean is 15 and there are 5 pieces of data in the set, multiply 15 * 5.  From there, find the sum of all of the pieces of data you do have.  Then, find the difference.  This will give you the missing piece of data that you are looking for.  Not to mention, this is a great way to figure out what kind of a score you need on your final to keep your 92% in math at the end of the year.  :o)

Good luck!
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thank you so much! just started Algebra 1 this year and drawling out for first test!