Question 61
The average of eleven numbers is 54. The average of the first four numbers is 48 and that of the next four numbers is 25% more than the average of the first four. The ninth number is 8 greater than the $$11^{th}$$ number and the tenth number is 4 greater than the $$11^{th}$$ number. What is the average of the $$9^{th}$$ and the $$10^{th}$$ numbers?
Solution
The sum of 11 numbers = 54*11 =594
Now sum of 1st four numbers = 48*4 =192
Now sum of next four numbers = 192(1.25)=240
Now we get sum of 9tj to 11th number = 594-240-192=162
a11 +a10+a9 =162
we get a11+8+a11+4+a11 =162
we get 3a11 = 150
a11=50
So a9+a10 =112
Therefore average =56
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