"I want forty-six oranges."
Translation:Dw i eisiau pedwar deg chwech oren.
The reason why is probably lost in the mists of time! But as explained in the notes, pump, chwech, cant all lose their last letter when used in front of a noun and become pum, chwe, can.
(Also, as explained in the notes there are some errors in making that change in this beta version of the course, but these will be fixed as soon as possible after the beta period finishes - no need to report the errors, as they are already identified.)
You may have made a mistake in another part if the sentence, in which case it would show a correction which would probably be the original version if the sentence with the chwech typo. As we explain in the notes, we cannot get rid of ...chwech oren until we can release the next version if the course.