Question

The sum of four numbers is 20 and their average is 5. When a fifth is added, the new average is 6. What is the fifth number?

Answer 1

Answer:

10

Step-by-step explanation:

We'll note the numbers as $x_{n}, x_{n+1} ...$

$x_1 + x_2 + x_3 + x_4 = 20\\Avg = 5\\\frac{x_1 + x_2 + x_3 + x_4 + x_5 }{5} = 6 = \frac{20 + x_5}{5}\\ 4 + \frac{x_5}{5} = 6\\\\frac{x_5}{5} = 2\\\x_5 = 10$

Answer 2

Answer:

10.

Step-by-step explanation:

The average of the 5 numbers =

(20 + x) / 5 = 6  where x is the 5th number.

20 + x = 30

x = 10

The fifth number is 10.