stonecold
Set S is given as S = {1,3,7,13,17,23,29,31,33,37,39,49,51}. In how many ways can three numbers be chosen from Set S such that the sum of those three numbers is 54?
A) five
B) four
C) three
D) two
E) zero
Great question, stonecold!!It illustrates a point I often make about the fact that the test-makers are
reasonable people. That is, they don't expect us to engage in long/hard/tedious/time-consuming calculations.
So when I started working on this question, I started looking for at least one set of 3 values that added to 54. After 20 seconds of this, I realized that,
IF I were able to find 3 such values, it would be difficult for me to find another set of 3 values, AND it would be super difficult to know when I had found ALL possible sets.
So, knowing what I know about the testmaker being reasonable, I abandoned my search of values that worked and started looking for other approaches.
That's when I realized that all of the values in the set were ODD integers.
Once I noticed this, I recalled what I know about the sums of ODD integers.
Since we know that ODD + ODD + ODD = ODD, we can see that it's impossible to select three ODD values from the given set that add to 54 (an EVEN number)
Answer: