An hour, we use "an" next to a word start with vowels letters like "a, e, i, o, u" and we use "a" to a word starts with consonant letters ( b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y,...
It is "An" hour, not a hour. The "an" comes before every vowel sound, not every vowel.
18250...
The hour you were born in...
There are 24 hours in a day so 10 days is 240 hours. 243 hours is 10 days and 3 hours.