number of triangles. That the 3! and the 7! have been transposed makes no difference, since
the order in which multiplication occurs is irrelevant. So the number of heptagons minus the
number of triangles is 120 – 120 = 0.9 . D
First think about this situation from the perspective, not of formulas you must know for
Math 2, but of simple common sense. Imagine yourself actually tossing a coin five times.
Does it seem relatively likely or unlikely that you’ll get heads twice or more? It’s relatively
likely—and that tells you something about the smart way to handle this question. When
simple reflection—plain old common sense—makes you think, “The event they’re asking for
has a pretty high probability of occurring,” let your reaction be, “It’d probably be easier to
figure out the probability of its not occurring and just subtract that probability from 1.” In
other words,
Now focus on the probability of getting zero or one heads, following the four steps discussed
in the body of this chapter. Recall that step one was to establish the sequence of events—in
this case, a series of tosses:Toss 1 Toss 2 Toss 3 Toss 4 Toss 5
Second, write out the different ways in which what you want to occur could occur. Suppose
that H means heads and T means tails. Keep in mind that, at this point, what you want to
occur is zero heads or one heads (H):Toss 1 Toss 2 Toss 3 Toss 4 Toss 5
One way: T T T T T
Another way: H T T T T
Another way: T H T T T
Another way: T T H T T
Another way: T T T H T
Another way: T T T T H