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.

There are 8 red tegu lizards in the reptile house at a local zoo. If the average length of the red tegu lizards is 10 inches, then how many of the red tegu lizards are exactly 10 inches long?

(1) None of the red tegu lizards are shorter than 10 inches.

(2) None of the red tegu lizards are longer than 10 inches.

When you modify the original condition and the question, there are avg=10, 8 variables, 1 equation(avg=10), which should match with the number of equations. So you need 7 equations. For 1) 1 equation, for 2) 1 equation, which is likely to make E the answer.

When 1) & 2), in 1), none shorter than= at least=min=avg -> standard deviation=0 -> the length of all 8 lizards is 10, which is sufficient.

In 2), none longer than= at most=MaAx=avg -> standard deviation=0 -> the length of all 8 lizards is 10, which is sufficient, which is sufficient. Therefore the answer is D. This type of question is a current trend found in GMAT Math.

-> For cases where we need 3 more equations, such as original conditions with “3 variables”, or“4 variables and 1 equation”, or “5 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 80% chance that E is the answer (especially about 90% of 2 by 2 questions where there are more than 3 variables), while C has 15% 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 E 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, C or D.

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