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.
A circle and a rectangle are there shown as above figure. If the area of the circle is 100π, what is the perimeter of the rectangle?
1) The one side length of the rectangle except the radius is 20
2) The area shaded is 200-25π
Attachment:
perimeter.jpg
As above the picture, suppose one side of the rectangle is a, which makes 1 variable(a). In order to match with the number of equations, you need 1 equation.
For 1) 1 equation, for 2) 1 equation, which is likely to make D the answer.
1) a=10 unique –> unique and sufficient.
2) a=10 unique -> unique and sufficient.
Thus, the answer is D.
Interesting, can you please elaborate though? I am not sure if I understood your approach completely.