We use "24 hours a day" to specify a length of time that something happens or someone does something. For example - "The shop is open 24 hours a day.". This is a set expression the same as "eight hours a day" or "seven days a week". It suggests an ongoing action or habit. But if we are talking about what a day is made up of we use "in" - "There are 24 hours in a day. " like "There are seven days in a week".

A day contains 24 hours. Therefore, there are 24 hours (contained) in a day.

"u" is not a word in English. Its use indicates sloppiness and laziness. Please use "you".