why did chunilal played a prank with bepin babu? what did Bepin babu did after knowing the prank?

Chunilal played a prank on Bepin Babu because once he had helped the latter in his hard times but when it was Bepin Babu's turn to help Chunilal, he forgot. However, when Bepin regained his memory, he wrote a letter to Chunilal apologizing for his behaviour.