Question

You are opening boxes of cereal one at a time looking for your favorite prize, a Captain America decoder ring, which is in 15% of the boxes. How many boxes would you expect to open, on average, until you find your favorite prize

Answer 1

6 or 7 boxes (I'd suggest you say 7 because it's rounding up)

Explanation:

15 out of 100 boxes contain the Captain America decoder ring.

3 out of 20 boxes contain the Captain America decoder ring.

1 out of roughly 7 boxes contain the Captain America decoder ring.

You need to open about 6 or 7 boxes (6.66...)