Look at this series: 80, 10, 70, 15, 60, 20,... What number should come next?

Answer: 50

Split the sequence into two interleaved subsequences:

• Positions 1, 3, 5, ...: 80, 70, 60, ... (subtract 10 each time)

• Positions 2, 4, 6, ...: 10, 15, 20, ... (add 5 each time)

The given sequence ends with 20 (position 6). The next term is position 7, which follows the decreasing subsequence: 60 − 10 = 50.