Nearly 30 percent of the answers ----- (be) wrong.

Nearly 30 percent of the answers _____ (be) wrong.
Nearly 30 percent of the answers _____ (be) wrong.
我知道答案是is,但下面这个句子(新目标八年级英语上册的第二单元2b)与这句很相似,但为什么不是三单?
Only fifteen percent of our students exercise every day .