A coin is flipped $10$ times. How many outcomes have exactly three heads? How many outcomes have at most three heads?

  • $\begingroup$ I suspect that we need to put slots between $10-3=7$ tails so that we end up choosing $3$ from these $7$ empty slots: $_T_T_T_T_T_T_T_$ $\endgroup$ – Camron Ignazio Jul 23 '13 at 3:05
  • $\begingroup$ Does $HHHTTTTTTT$ fit in that pattern? $\endgroup$ – Daniel Fischer Jul 23 '13 at 3:07
  • 1
    $\begingroup$ Do you know about binomial coefficients? For example, do you know what ${10\choose 3}$ means? $\endgroup$ – Potato Jul 23 '13 at 3:08
  • $\begingroup$ The number of ways $3$ things can be chosen from $10$ things. $\endgroup$ – Camron Ignazio Jul 23 '13 at 3:09

Imagine writing down a sequence of length $10$, made up of the letters H and/or T, to indicate what happened on your tosses.

Exactly $3$ heads happened precisely if the sequence has exactly $3$ H (and therefore $7$ T).

There are $\binom{10}{3}$ ways to choose where the $3$ H will go, so there are $\binom{10}{3}$ such sequences.

For at most $3$ heads, do the same sort of thing for $0$ H, $1$ H, $2$ H, and $3$ H, and add up.


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