I would expect 85% as answer if you believe “In penile-vaginal intercourse with an HIV-infected partner, a woman has an estimated 0.1% chance of being infected, and a man 0.05%. (1 in 2000) Condoms further reduce the risk by 85%” as your link indicates.
What do you mean by “prevent”? P(contagion|no condom)/P(contagion|condom) << 1? If so, does 0.15 count as << 1?
I would expect 85% as answer if you believe “In penile-vaginal intercourse with an HIV-infected partner, a woman has an estimated 0.1% chance of being infected, and a man 0.05%. (1 in 2000) Condoms further reduce the risk by 85%” as your link indicates.