Write the next term in the following series: 15, 10, 45, 20, 135, 30, _______

Pattern: the sequence alternates between two subsequences — one that is multiplied by 3 and one that increases by 10.

• Subsequence for positions 1, 3, 5, ...: 15, 45, 135 — each term is previous × 3, so next = 135 × 3 = 405.

• Subsequence for positions 2, 4, 6, ...: 10, 20, 30 — each term increases by 10.

Therefore the 7th term (which belongs to the first subsequence) is 405. Answer: 405