The average of twelve positive numbers is 18. If the average of the first four numbers is 12 and that of the last four numbers is 30. What is the average of the remaining four numbers?

\$"Average" = "sum of set of numbers" / "amount of numbers in set"\$

Given that,

Average of twelve positive numbers = 18

Average of the first four numbers = 12

Average of the last four numbers = 30

Let x be the average of the remaining numbers

\$18 = (12 + x + 30)/3\$

Multiplying 3 on both sides

\$18 \times 3 = \cancel3^1 \times (12 + x + 30)/\cancel3^1\$ ⇒ \$54 = x + 42\$

⇒ \$x = 54 - 42 = 12\$

So, the average of the remaining numbers is 12