10,4,7
The above set 4 numbers have a mean of 8
what could the mystery number

11

Step-by-step explanation:

To calculate the mean you add all the numbers and divide by the number of numbers. There are 4 numbers in this set so we will divide by 4.

Let x be the unknown number,

(10+4+7+x)/4=8(mean)

To solve for x, we first mutliply both sides by 4.

To simplify the left side we can add 10+4+7 and get 21 where now our equation is 21+x=32. Subtract 21 on both sides and you will get 11 which is our answer.

Plug-in 11 and solve for the mean to verify where we do indeed get 8 as the mean.

i would have to say it is c.