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.


Part of 27 pebbles is removed from on the desk by 2 persons in turn. After removing by 2 persons, is the number of the remained pebbles an odd number?
1) One person removed 4 pebbles more than the other person removed.
2) The sum of the number of pebbles removed is 8


When you modify the question in the original condition, it becomes 27-(a+b)=odd?. Then there are 2 variables(a,b), which should match with the number of equations. So you need 2 equations. For 1) 1 equation, for 2) 1 equation, which is likely to make C the answer.
When 1) & 2), a=2, b=6 are derived, which makes 27-8=19=odd. It is yes and C is the answer. However, since this is an integer(hidden) question, which is one of the key questions, apply the mistake type 4. For 2), since a+b=8, 27-8=19=odd, which is yes and sufficient.
For 1), in a=b+4, a+b=b+4+b=2(b+2)=even is derived, which is yes and sufficient. Therefore, the answer is D. This is the mistake type 4(B) question.