Translation:It is twenty-five minutes past seven.
Short answer: five.
Long answer: pump is "five", but it becomes pum before a noun (like chwech becomes chwe before a noun). So "five minutes" is "pum munud". The 'n in mae hi'n is a shortening of yn - yn causes nouns and adjectives (but not verbs) to mutate (soft mutation), therefore pum becomes bum.
Summary: bum is a shortened and mutated form of pump.