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.
The p[oints A, B, C and D are on a number line, not necessarily in that order. If the distance between A and B is 18 and the distance between C and D is 8, what is the distance between B and D?
1) The distance between C and A is the same as the distance between C and B
2) A is to the left of D on the number line
Transforming the original condition and the question, we have multiple variables since the position of A,B,C,D is not fixed. Since there is only 1 equation each in 1) and 2), there is high probability that E is the answer.
Using both 1) and 2), BD=1, 17 thus the answer is not unique. Therefore the answer is E.
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