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The average of six observations is 15. The average of first three among them is twice the average of the last three. What is the sum of the last three observations?
Average of 6 numbers = 15
Sum of 6 numbers = $$\frac{1st + 2nd +3rd +4th + 5th + 6th}{6} = 15$$ Eq.(i)
It is given ,the average of first three among them is twice the average of the last three.
(1st + 2nd + 3rd) = 2(4th +5th + 6th) Eq.(ii)
Put Eq.(ii) in Eq.(i).
3(4th + 5th + 6th) = 90
(4th + 5th + 6th) = 30
So , the answer would be option d)30.
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