"The suit costs forty-three pounds."
Translation:Mae'r siwt yn costio pedwar deg tri punt.
The least common mutation in Welsh is the aspirate one when the letters 'C', 'P' and 'T' add an 'h' .
The number 'tri' causes this mutation to happen, also compound numbers containing it.
Thus the completely grammatically correct answer is the one with the mutation.
However this mutation is slowly disappearing from the language and thus the version without the mutation is very commonly heard.
Really, I would want to say pedwar deg tri o bunnoedd here if I was using the decimal system of numbers - the problem vanishes then anyway!. I'll ask an expert...
Edit - my expert said that of the various decimal options plus a singular, they would (if forced, and through gritted teeth) choose to go with pedwar deg tair punt - since punt is feminine.