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25 Sep 2019, 05:34
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A
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
75%
(hard)
Question Stats:
53% (02:06) correct
47%
(02:06)
wrong
based on 217
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What is the first term of an arithmetic progression of positive integers ?
(1) Sum of the squares of the first and second term is 116.
(2) The seventh term is divisible by 10.
(1) Sum of the squares of the first and second term is 116.
(2) The seventh term is divisible by 10.
Archit3110
Major Poster
25 Sep 2019, 08:39
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Bunuel
#1
Sum of the squares of the first and second term is 116.
only possiblity ; 4^2+10^2 ; 116
sufficient
#2
The seventh term is divisible by 10
insufficient
to determine as series progression not know
IMO A
25 Sep 2019, 13:26
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Bunuel
Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.
The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.
Since an arithmetic progression has two variables, which are its first term and its difference and 0 equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.
Conditions 1) & 2)
Assume \(a_1\) is its first term and \(a_2\) is its second term.
If \(a_1 = 4\) and \(a_2 = 10\), then we have a difference \(6\) and the seventh term \(a_7 = 4 + 6*6 = 40\) which is divisible by \(10\), \(a_1\) is a positive integer and then answer is 'yes'.
If \(a_1 = -4\) and \(a_2 = -10\), then we have a difference \(-6\) and the seventh term \(a_7 = -4 + 6*(-6) = -40\) which is divisible by \(10\), \(a_1\) is a negative integer and then answer is 'no'.
Since both conditions together do not yield a unique solution, they are not sufficient.
Therefore, E is the answer.
Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
