Question 442917
The average of a set of values is the sum of the values divided by the number of values. In this case (using x as the unknown value/test score), the values are 87, 63, 79, 71, 96, and x. The number of values/test scores is six, and you know that the average is 78. Therefore, you can use the equation {{{78 = (87 + 63 + 79 + 71 + 96 + x)/6}}}. Next, solve for x:

Multiply both sides of the equation by 6: {{{468 = 87 + 63 + 79 + 71 + 96 + x}}}

Simplify the right side of the equation by adding the values/test scores together: {{{468 = 396 + x}}}

Subtract 396 from both sides to isolate x: {{{72 = x}}}

Therefore, the missing test score must be 72.