The average(arithmetic mean) of nine numbers is 9. When a tenth number is added the average of the ten numbers is also 9. What is the tenth number ?

A) 0

B) 9/10

c) 10/9

d) 9

e) 10 The answer is D.The avg of 9 nos is 9 => Total of 9 nos = 9*9 = 81

For the avg to remain 9 when another number, say x, is added, the following equation should hold true: (81 + x)/10 = 9

=> x = 9

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