Find the next term (?) in the given series:1, 3, 6, 10, 15, ?, ...

The correct answer is Option (2) → 21

The differences between neighbouring terms go up by 1 each time, so the next difference is 6.

• Differences: \(3-1=2\), \(6-3=3\), \(10-6=4\), \(15-10=5\)

• Next term: \(15+6=21\)

These are the triangular numbers \(1, 1+2, 1+2+3, \ldots\); the sixth is \(1+2+3+4+5+6=21\).