Question

how did harold come to know that his father was a boxer

Answer 1

Jerry Fisher the trainer of Mr. bramble told Harold that Mr. bramble is a boxer because he was angry from Mr. bramble as he refused to fight with American Murphy.
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Answer 2

Mr. Jerry Fisher didn't take Bill's withdrawal from the contest very kindly . At that moment he hated Bill the most .He wanted to take revenge . So , he disclosed Bill's real identity to his son Harold . He told that Bill is not a commercial traveller but a professional boxer and was popularly known as young porky.

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