The total amount of money in the account at the end of the 20th day is 5230176601 cents
Brian deposited 1 cent on the first day with an additional 3 cents, then 9 cents on the second day, 27 cents on the third day and so on, this form a geometric series.
A geometric series is a series which has a constant ratio between consecutive terms. This ratio is called the common ratio, r.
Now, the total amount of money, since we add up till the 20 th day, is the geometric series,
s = 1 + 3 + 9 + 27 + ...
s = 1 + 3¹ + 3² + 3³ + ... + 3²⁰
Now the common ratio of the series is 3.
Now the total sum is the sum of the series which is
S = a(rⁿ - 1)/(r - 1) where
So, substituting the values of the variables into the equation, we have
S = a(rⁿ - 1)/(r - 1)
S = 1(3²¹ - 1)/(3 - 1)
S = (3²¹ - 1)/2
S = (10460353203 - 1)/2
S = 5230176601 cents
So, the total amount of money in the account at the end of the 20th day is 5230176601 cents
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