(1) Observe the law of this series: 5, 6, 2, 4, four numbers appear repeatedly, and the cycle segment is 4, which is the key to solving the problem.

(2) 129÷4=32 (times) ... 1 (pieces) ... That is to say: 5, 6, 2, 4, four numbers appear repeatedly for 32 times, leaving 1, which is the first number 5.

(3) The sum of129 numbers: (5+2+6+4) × 32+5 = 545.