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The median of a set with odd # of terms is just a middle term (when ordered in ascending/descending order).
The median of a set with even # of terms is the average of two middle terms (when ordered in ascending/descending order).
Next, one of the most important properties of evenly spaced set (aka arithmetic progression):
In any evenly spaced set the arithmetic mean (average) is equal to the median and can be calculated by the formula \(mean=median=\frac{a_1+a_n}{2}\), where \(a_1\) is the first term and \(a_n\) is the last term. Given the set \(\{7,11,15,19\}\), \(mean=median=\frac{7+19}{2}=13\).
(1) All members of \(S\) are consecutive multiples of 3. This statement says that \(S\) is an evenly spaced set, thus its mean equals to its median. Sufficient.
(2) The sum of all members of \(S\) equals 75. Clearly insufficient, consider two sets \(\{25, 25, 25\}\) and \(\{0, 0, 75\}\).
The median of a set with odd # of terms is just a middle term (when ordered in ascending/descending order).
The median of a set with even # of terms is the average of two middle terms (when ordered in ascending/descending order).
Next, one of the most important properties of evenly spaced set (aka arithmetic progression):
In any evenly spaced set the arithmetic mean (average) is equal to the median and can be calculated by the formula \(mean=median=\frac{a_1+a_n}{2}\), where \(a_1\) is the first term and \(a_n\) is the last term. Given the set \(\{7,11,15,19\}\), \(mean=median=\frac{7+19}{2}=13\).
(1) All members of \(S\) are consecutive multiples of 3. This statement says that \(S\) is an evenly spaced set, thus its mean equals to its median. Sufficient.
(2) The sum of all members of \(S\) equals 75. Clearly insufficient, consider two sets \(\{25, 25, 25\}\) and \(\{0, 0, 75\}\).
Answer: A
Don't mean to sound picky but is this sentence correct? "evenly spaced set (aka arithmetic progression)" don't evenly spaced sets include geometric progressions and arithmetic progressions?
The median of a set with odd # of terms is just a middle term (when ordered in ascending/descending order).
The median of a set with even # of terms is the average of two middle terms (when ordered in ascending/descending order).
Next, one of the most important properties of evenly spaced set (aka arithmetic progression):
In any evenly spaced set the arithmetic mean (average) is equal to the median and can be calculated by the formula \(mean=median=\frac{a_1+a_n}{2}\), where \(a_1\) is the first term and \(a_n\) is the last term. Given the set \(\{7,11,15,19\}\), \(mean=median=\frac{7+19}{2}=13\).
(1) All members of \(S\) are consecutive multiples of 3. This statement says that \(S\) is an evenly spaced set, thus its mean equals to its median. Sufficient.
(2) The sum of all members of \(S\) equals 75. Clearly insufficient, consider two sets \(\{25, 25, 25\}\) and \(\{0, 0, 75\}\).
Answer: A
Don't mean to sound picky but is this sentence correct? "evenly spaced set (aka arithmetic progression)" don't evenly spaced sets include geometric progressions and arithmetic progressions?
An evenly spaced set is one in which the gap between each successive element in the set is equal.
A geometric progression is NOT an evenly spaced set. For example, 2, 4, 8, 16, 32, ... is NOT evenly spaced because the gap between each successive element in the set is NOT equal.
Bunuel what about negative multiples?? what if the set is {-6,-3,0,3} is this not consecutive multiples?? in this case mean=-2 and median is -1.5 I think Statement 1 should say positive consecutive multiples? OR is that in GMAT multiples of any no. are always taken as positive multiples?? thanks
Bunuel what about negative multiples?? what if the set is {-6,-3,0,3} is this not consecutive multiples?? in this case mean=-2 and median is -1.5 I think Statement 1 should say positive consecutive multiples? OR is that in GMAT multiples of any no. are always taken as positive multiples?? thanks
We can consider negative integers as multiples too. For example, -4 is a multiple of 2.
The problem is that the mean of {-6,-3,0,3} is -1.5, not -2.
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I do not understand how the answer is "A." It depends on how many "members" we talk about.
(3+6+9)/3 = 6 (mean) and median 6 (3+6+9+12)/3 = 10 (mean) and 7.5 median
Therefore, I picked "C."
The mean of 3, 6, 9, and 12 is (3+6+9+12)/4 = 7.5. As it'd given in the solution: in ANY evenly spaced set the arithmetic mean (average) is equal to the median.
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66% (00:40) correct 
