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

The average of 30 numbers is 15. The average of the first 18 number is 10 and that of next 11 numbers is 20. The last number is :
[A]50
[B]52
[C]56
[D]60

50
Let the last number be x.
According to the question,
18 \times 10 + 11 \times 20 + x = 30 \times 15
=> 180 + 220 + x = 450
=> 400 + x = 450
=> x = 450 – 400 = 50
Hence option [A] is correct answer.

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