Question
Fifteen numbers have an average of 70. The average of
the first seven is 60, and the average of the last five is 63. The eighth number is 50% of the ninth, while the tenth number is 100% more than the ninth. What is the ninth number?Solution
Let ninth number is x Eighth number = 0.50x Tenth number = 2.00x According to question; x + 0.50x + 2.00x = 15 × 70 – 7 × 60 – 5 × 63 = 315 Or, 3.5x = 315 Or, x = 90
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