"It's five minutes past seven."

Translation: Mae hi'n bum munud wedi saith.

[MatthewSke1]

Why does pump change to bum, not bump here?

[ibisc]

Pump normally drops the last letter before nouns. So do chwech and cant:

• Pum cath - five cats

• Chwe ffenestr - six windows

• Can punt - a hundred pounds (or cant o bunnau)

Not always the case in informal usage, though.

[MatthewSke1]

Thank you. That explains it.