The mean of five numbers 3,7,9,12, and x is 8. find the value of x

  " x = 9 "  .

Explanation:

To calculate the mean:

The mean = 

     {the sum of all the values in the number set} ÷ 

     {the number of values in the data set} ;

 To solve for "x" ;

Given that the "mean" is:  "8" ;

            8  =  { 3 + 7 + 9 + 12 + x }  /  5  ;
 
    ↔  { 3 + 7 + 9 + 12 + x }  /  5   =  8  ;

            →  { 3 + 7 + 9 + 12 + x }  = 8 * 5  ;

            →  { 31 + x } = 40  ;

            ↔   x + 31  = 40  ;

       Subtract "31" from EACH SIDE of the equation; to isolate "x" on one side of the equation; and to solve for "x" ;

            →   x + 31 − 31  =  40 − 31  ;

                       →  x = 9  .