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adityakaregamba

Bunuel
Official Solution:


20 rolls of a fair six-sided die produced the following results:



What is the probability that one additional rolls will increase the current average (arithmetic mean) score?



A. \(\frac{1}{6}\)


B. \(\frac{1}{3}\)


C. \(\frac{1}{2}\)


D. \(\frac{2}{3}\)


E. \(\frac{5}{6}\)


The current mean score is \(\frac{1*4 + 2*3 + 3*5 + 4*2 + 5*2 + 6*4}{20} = \frac{67}{20} = 3.35\). Therefore, only a roll of 4, 5, or 6 will increase the mean score. Thus, the probability is \(\frac{3}{6}=\frac{1}{2}\).

Answer: C.

Hope it's clear.
­How did you came to conclusion that only 4,5,6 will increase the mean ? what about 1 ,2,3 ?
­
The current average is 3.35. To increase the average, the new data point must exceed the current average, hence it can only be 4, 5, or 6. Consider this analogy: suppose the average age of students in a class is 25 years. Any new student added to increase the average age must be older than 25 years. If the new student is 25 years old, the average remains unchanged, and if the new student is younger, the average will decrease.

Hope it's clear.
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