MathRevolution
[Math Revolution GMAT math practice question]
For an integer n greater than 1, the set S
n={ kn | k is an integer greater than or equal to 1} is defined. What is the value of n?
1) 2 belongs to the set S
n.
2) 4 belongs to the set S
n.
\(n \geqslant 2\,\,\,\operatorname{int}\)
\({S_n} = \left\{ {\,k \cdot n\,\left. {} \right|\,\,\,k \geqslant 1\,\,\operatorname{int} } \right\}\)
\(? = n\)
Let´s understand what each S_n means:
\({S_2} = \left\{ {\,2k\,\,\left. {} \right|\,\,\,k \geqslant 1\,\,\operatorname{int} } \right\} = \left\{ {2,4,6,8, \ldots } \right\}\,\,{\text{ = }}\,\,{\text{positive}}\,\,{\text{even}}\,\,{\text{integers}}\)
\({S_3} = \left\{ {\,3k\,\,\left. {} \right|\,\,\,k \geqslant 1\,\operatorname{int} } \right\} = \left\{ {3,6,9,12, \ldots } \right\}\,\,{\text{ = }}\,\,{\text{positive}}\,\,{\text{multiples}}\,\,{\text{of}}\,\,3\)
\({S_4} = \left\{ {\,4k\,\left. {} \right|\,\,\,k \geqslant 1\,\operatorname{int} } \right\} = \left\{ {4,8,12,16, \ldots } \right\}\,\,{\text{ = }}\,\,{\text{positive}}\,\,{\text{multiples}}\,\,{\text{of}}\,\,4\)
and so on...
\(\left( 1 \right)\,\,\,2 \in {S_n}\,\,\, \Rightarrow \,\,\,\,n = 2\,\, \Rightarrow \,\,\,\,{\text{SUFF}}.\)
\(\left( 2 \right)\,\,\,4 \in {S_n}\,\,\, \Rightarrow \,\,\,\,n = 2\,\,\,{\text{or}}\,\,\,n = 4\,\,\,\, \Rightarrow \,\,\,\,{\text{INSUFF}}.\)
The right answer is therefore (A).
This solution follows the notations and rationale taught in the GMATH method.
Regards,
Fabio.