Three numbers are 4, 6 and one unknown number. The mean of the three numbers is 6. What is the unknown number?

Explication

A mean of 6 across 3 numbers means the total must be 18, and the two known numbers already contribute 4 + 6 = 10, so the unknown number is 18 - 10 = 8. Answering 6 simply repeats the mean, which would leave the total at 16 and drag the mean below 6.