If you flipped a fair coin three times, what is the probability that it would come up "heads" at least once? %

If you flip the coin three times, you have four possible number of heads:

A: No heads. all coins land on tails

B: One heads. one coin lands on heads, two coins land on tails

C: Two heads. two coins land on heads, one coin lands on tails

D: Four heads. all coins land on heads.

You're interested in the union of the latter three events (one, two or all heads). In this case, it is smarter to compute the probability of the remaining event (no heads), and compute the probability of the other three by completing to 1: we have

We have , because we want three consecutive flips to land on tails, and each flip has a probability of 0.5 to land on tails.

So, at least one coin lands on heads with probability

The answer is that there are are 35 acres in Happyland. The population is 553 Happys, and 15.8 of them live in an acre. 553 total population divided by 15.8 Happys is 35 acres. To check the work, multiply 35 x 15.8 and you get 553.