WBCS Preliminary Examination 2024 — General Studies · Question 37 of 200

Answer: (B) 30

The terms are products of consecutive numbers, $n(n+1)$: $1 \times 2 = 2$, $2 \times 3 = 6$, $3 \times 4 = 12$, $4 \times 5 = 20$, so the next is $5 \times 6 = 30$. The differences confirm it: 4, 6, 8, and therefore 10 next ($20 + 10 = 30$). Such numbers ($n^2 + n$) are called pronic or oblong numbers, and the series continues 42, 56, 72. In any number series, first write down the differences; if they form a simple pattern (here, consecutive even numbers) the answer follows at once.