Quantitative Aptitude

$S_n = (n/2)[2(5) + (n-1)4] = (n/2)[10 + 4n - 4] = n(2n + 3) = 2n^2 + 3n = 324$. $2n^2 + 3n - 324 = 0$. Factors: $2 \times 324 = 648 = 27 \times 24$. $(2n + 27)(n - 12) = 0 \implies n = 12$.