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 tactic 6: replace variables with numbers

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T O P I C    R E V I E W
mingshum Posted - 03/11/2009 : 16:33:26
In my Barron's SAT prep book, the tactic of replacing variables with easy-to-use numbers was used in this example:

If the sum of four consecutive odd integers is s, then, in terms of s, what is the greatest of these integers? so using the numbers 1,3,5,7 = s= 16 and the largest integer is 7 so the choices were:

a) (s-12)/4 b) (s-6)/4 c) (s + 6)/4 d)(s +12)/4 e) (s +16)/4

d) is the only correct answer when s =16 and 7 is the largest integer.

Why doesn't this tactic work with this problem? If the sum of five consecutive integers is S, what is the largest of those integers in terms of S? I used 1,2,3,4,5(answers C,D were correct) and also 2,3,4,5,6 (answers B and E were correct.)?
3   L A T E S T    R E P L I E S    (Newest First)
sahsjing Posted - 03/13/2009 : 22:21:41
quote:
Originally posted by mingshum

Thank you Skeeter...

So under what conditions would you use this tactic of replacing variables with numbers?


Use your own example: 1, 3, 5, 7. S = 16
(s+12)/4 = 7.

mingshum Posted - 03/13/2009 : 16:19:37
Thank you Skeeter...

So under what conditions would you use this tactic of replacing variables with numbers?
skeeter Posted - 03/11/2009 : 18:55:02
the answer choice would be different ...

for five consecutive integers ...

S = x + (x+1) + (x+2) + (x+3) + (x+4)

S = 5x + 10

(S - 10)/5 = x

(S - 10)/5 + 4 = (x + 4)

(S + 10)/5 = (x + 4)

in the example problem, the correct answer choice was "engineered" to work with the sum of four odd consecutive integers, not five consecutive integers.

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