I think the '6-coin-problem' is more complicated than that - the first coin must be either 5 cents or 10 cents (if it is 20 cents, the next coin cannot be 'higher'), so the probability is 3/6 (which is 1/2). If the first coin is 5 cents, the second coin MUST be higher, but if it is 10 cents, the probability of the second coin being higher is 3/5. You surely are required to combine these probabilities in some way to answer the question?