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.
In what year was Ellen born?
(1) Ellen’s brother Pete, who is 1 1/2 years older than Ellen, was born in 1956.
(2) In 1975 Ellen turned 18 years old.
In the original condition, you need to figure out the year Ellen was born. So, there is 1 variable, which should match with the number of equations. So you need 1 equation. For 1) 1 equation, for 2) 1 equation, which is likely to make D the answer.
For 1), since it’s 1/2 from 1 and 1/2 years older, it is not unique and not sufficient. That is, if the older brother Pete was born in December 1956, Ellen should have been born in 1958. If Pete was born in January 1956, Ellen should have been born in 1957.
For 2), it makes Ellen born in 1957 since she turned 18 in 1975, which is unique and sufficient. Therefore, the answer is B.
-> For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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