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The average of 30 numbers is 15. The average of the first 18 numbers is 10 and that of the next 11 numbers is 20. What is the last number?

March 16, 2024

Solution:

Let the last number be x.
According to the question,
18 x 10 + 11 x 20 + x
= 30 x 15
⇒ 180 + 220 + x = 450
⇒400 +x= 450

⇒x=450-400=50

Then the answer is 50