There is a soft mutation after yn / 'n, (but verbs don't mutate). We say:

There is also a soft mutation after i:
Mae hi'n chwarter i dri.
Mae hi'n bum munud i bedwar.
Mae hi'n ddeg munud i ddau.

There are two counting systems in Welsh - the traditional system and the modern system. The traditional system is always used when telling the time, and very often when talking about people's ages. The simpler modern system was developed to facilitate the teaching of Maths through the medium of Welsh. It is the more common of the two systems now, especially amongst younger people.