Find the number that will come next in the following series.
12, 15, 21, 24, 30, 33, ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 39
Each term is the one before it plus the sum of its own digits:
\(12+(1+2)=15\)
\(15+(1+5)=21\)
\(21+(2+1)=24\)
\(24+(2+4)=30\)
\(30+(3+0)=33\)
\(33+(3+3)=39\)
So the next number is 39. (The steps +3, +6, +3, +6, ... give the same answer.)