[Math] A coin is tossed $5$ times. How many possible outcomes contain AT MOST $3$ heads

Question

There are $32$ possible outcomes in total when a coin is tossed $5$ times. I have found that there are 10 possible outcomes that contain exactly$ 3$ heads by using $5C3=5!/3!2!$,
but how do I find out how many possibilities contain at most $3$ heads?
Thanks!

Answer 1

This would be modeled with a sum of binomial coefficients. If you want precisely $k$ events to occur out of $n$ events, where event can only have success or failure, then the number of corresponding outcomes is $n$ choose $k$, i.e. $\binom n k$.

If you want at most some number of events, you can sum over the corresponding $k$ values, the $k$ values you deem valid. For example, if you want at most $3$ successes, you take the sum of $\binom n k$ for $k=0,1,2,3$.

In your case, $n=5$ and thus your result is

$$\sum_{k=0}^3 \binom 5 k$$