Acoin favors heads when tossed and there are 15 heads in 18 tosses

This is not correct.  You might think you are "on a roll" but the probability of heads on any toss is just as likely as the probability of tails because each coin toss is an independent event, meaning, two events related in such a way that knowing about the occurrence of one event has no effect on the probability of the the other event.  This would hold true if you have tossed these 15 heads and you believe that tails "are due".  It is an independent event.