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You may like to read the article on Morse code before attempting this question. Morse's letter analysis was done over 150 years ago, so might there be a better allocation of symbols today?
Florence Nightingale may be well known for her role as a nurse, but she was also an excellent mathematician, collecting and analysing data to help improve hospital conditions.
This problem offers you two ways to test reactions - use them to investigate your ideas about speeds of reaction.