The problem says that they wake up in the same day (but in different time). But what if some dummies will have to wait until the midnight passes? I can output the time then, but it will lead to misunderstanding. Is it guaranteed, that they all shall be able to drink in the same day?

No! You should check this situation, too! Some of them maybe will be able to drink tea after midnight! ) good luck!)

What shall I use for these formulas. Is float or double precision enough for that, or perhaps I should use fractional arifmetics (represent fractions nominator and denominator)?

I got AC with double precision!)