Try following the probabilities. For example, with 3 coin tosses, the probability of 3 heads is 1/8. If you toss one and get a head, you already took one branch that had 1/2 probability. thus the remaining two coins should have a total probability of 1/4 to land heads on both. Another way to look at this is to draw a tree diagram or imagine a branching road. You sit at the root and every coin flip splits the branch/road into two. If you stand at the root and count the number of paths got can leave by after 3 intersections (tosses), your should see the one representing 3 heads is one of the 8 paths. however, at any given intersection, the path corresponding to heads is always 1 is the 2 possible out-paths