“People were 60”
“I attended a lecture yesterday, and people attending were 60,” is wrong. It should be, “I attended a lecture yesterday and there were 60 attendees,” or “there were 60 people there.”
‘昨日講演会に行きました。60名参加していました。’という文章です。数字は名詞の前にし、60 people や、60 attendees とします。 People were 60.では、‘人々は60才でした。’という意味になります。