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Solution

50-50.

Since you have 28 and I have 27 coins, it is certain that you have either tossed more heads than I have, or more tails. (You can't have done both.) Since these are the only two options, and since there is nothing that favours a coin to end as tails rather than heads, the probability that you toss more heads than me must be 50-50 - a simple answer to what at first sight looks like a difficult probability question.