Question

A leprechaun places a magic penny under a​ girl's pillow. The next night there are 2 magic pennies under her pillow. The following morning she finds four pennies.​ Apparently, while she sleeps each penny turns into two magic pennies. The total number of pennies seen under the pillow each day is the grand​ total; that​ is, the pennies from each of the previous days are not being stored away until more pennies magically appear. How many days would elapse before she has a total of more than ​$5 billion​? ​(Proceed by trial and​ error.)

nothing days
​(Type a whole​ number.)

Answer 1

34

Explanation:

Since the pennies need to be kept under the pillow in order to be doubled, we're really looking for the day the girl will wake up with 5 billion pennies under her pillow.

The general formula of that geometric sequence will be: t = 1 + 2^(n-1)

We need to find the value of n when t will be over 5,000,000,000.

So, the equation becomes:

5,000,000,001 = 1 + 2^(n-1)

2^(n-1) = 5,000,000,000

log(2^(n-1)) = log(5,000,000,000)

(n-1) * log(2) = log(5,000,000,000)

n-1 = log(5,000,000,000) / log(2)

n-1 = 22.25

n = 33.25

Since it has to be complete days, and we have to reach over 5 billion, then we round it up to 34.