When he said that, everyone burst out laughing at once.
Tips
usage
一哄 lives mostly in three idioms about a crowd moving as one, usually without thinking: 一哄而上 "to pile in all at once", 一哄而散 "to scatter all at once", 一哄而起 "to rush into action all at once".
mistakes
哄 here is hōng, the roar of a crowd, as in 哄堂大笑. It is not hǒng "to coax": in 孩子一哄就不哭了 "the child stops crying as soon as you soothe him", the two characters are separate words.
