The arithmetic mean of three numbers is 8. Find the third number if the first is 12.4 and the second is 8.2.

Let the third number be x. Knowing the first number – 12.4, the second number – 8.2, the arithmetic mean of three numbers – 8, we can draw up an equation and solve it.
Let’s see what happens:
(12.4 + 8.2 + x) / 3 = 8;
(20.6 + x) / 3 = 8, we solve by the method of multiplication crosswise:
20.6 + x = 8 * 3;
20.6 + x = 24, x is found as an unknown term, that is:
x = 24 – 20.6;
x = 3.4.
If x = 3.4, then the third number is 3.4.
Answer: 3.4.