In theory the numbers 5 and 6 in Welsh (pump and chwech) contract in front of a noun.
Pump ------> Pum
Chwech -------> Chwe
Chwe then induces an aspirate mutation in the following noun.
Therefore 'six feet' should be 'chwe throedfedd'
However given the need to teach Maths and Science through the medium of Welsh, this has started to become simplified to 'chwech troedfedd'
Either are marked correct in this exercise.