P

Parvesh Kanani • 2.84K Points
Extraordinary Math 

Q. The average of 30 numbers is 12. The average of the first 20 of them is 11 and that of the next 9 is 10. The last number is

  • (A) 60
  • (B) 45
  • (C) 40
  • (D) 50
Correct Answer - Option(D)
Explanation by: Parvesh Kanani 
Given , Sum of 30 numbers = 30 × 12 = 360
Sum of the first 20 numbers = 20 × 11 = 220
and Sum of the next 9 numbers = 9 × 10 = 90
∴ Last number = Sum of 30 numbers – Sum of the first 20 numbers – Sum of the next 9 numbers
Last number = 360 – 220 – 90
Last number = 360 – 310 = 50

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