The first time: ①+② is heavier than ③+④, which means ①+② is heavier, and ③+④ is at least one lighter. Similarly, the second time: ⑤+⑤ is lighter than ⑤+⑧; ⑦+⑧ Both are heavy, and at least one of ⑦+⑦ is light. The third time: ①+③+⑤ is as heavy as ②+④+⑧.

Because ① ② ⑧ Heavy and ③ ⑧ can't both be heavy, ④ is light, then ③ must be heavy, so ⑤ is light.