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Harshani
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Bunuel
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gmatophobia
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why did we do this " Mean + 1.5 * SD = 13.50 + 12 = 25.50 " ?
gmatophobia


Mean = \(\frac{2 + 3+ 3+4+6+8+12+15+18+21+24+28}{12} = \frac{144}{12} = 12\)

SD = 9

1.5*SD = 1.5 * 9 = 13.50

Mean + 1.5 * SD = 13.50 + 12 = 25.50

How many plants have weight over 25.50 ? ⇒ Only 1, the one which has a weight of 28

Option B
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Hi MakB,

The question asks how many of the numbers (the plants) are more than 1.5 standard deviations above the mean.

As gmatophobia showed above, the mean is 12 and the standard deviation is 9. "1.5 standard deviations" means 1.5 times the standard deviation, or 1.5(9) = 13.5

Thus, "1.5 standard deviations above the mean" means 13.5 above 12. So we add: 13.5 + 12 = 25.5

The only number more than 25.5 (i.e., the only number "more than 1.5 standard deviations above the mean") is 28, so the answer is 1 (choice B).

Hope that's helpful!
MakB
why did we do this " Mean + 1.5 * SD = 13.50 + 12 = 25.50 " ?

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