Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.
Is the positive two-digit integer N less than 40 ?
(1) The units digit of N is 6 more than the tens digit.
(2) N is 4 less than 4 times the units digit.
There are 3 variables (n,a,b) and 1 equation (n=10a+b) in the original condition, 2 equations in the given conditions, making it likely that (C) will be our answer.
Looking at the conditions,
For 1, n=17,28,39<40 answers the question yes, and is therefore sufficient.
For 2, 10a+b=4b-4, 10a+4=3b, and this answers the question 'yes' for n=28<40. This is sufficient as well, making the answer seem like (D).
However, this is a logic question with commonly made mistake type 4(B).
Hence, (C) also becomes the answer but if we apply integer commonly made mistake type 4, the answer still becomes (D).
For cases where we need 2 more equation, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.
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