I can read French but I cannot write it, so please pardon me
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The 50% possibility of transmission is on every puppy. It means that it is like throwing a coin in the air and seeing if it exits a face or the other. Every throw has 50% for each face BUT it does not depend on the other throws. On 10 throws, you could have 3 face ONE and 7 face TWO, it does not mean that the probability is 30/70, it is 50/50 but every launch is independent. The same goes for the puppies... I hope I have been clear enough ^^.