CareerCup

100% interest rate means that the amount that you have in the bank doubles after each year: $1 at the beginning, 2 * $1 after one year, 2 * 2 * $1 after two years, ..., 2^n * $1 after n years = 15B . Therefore log_2(15B) = n.

Think of it is a binary tree, interest is 100%, means every dollar becomes double, so essentially answer is height of binary tree where number of leaves is 15 billion.

With 100% interest rate, money is doubling every year.

1, 2, 4, 8, ..... x

So x = 2^k
x = 15 billions

So 2^k = 15 billions
k = log(15 billions) -log with base 2

ln(15 billion) - ln(2)

this cannot be a interview question. It's middle school math. Too easy, not much knowledge involved

What could this interview question possibly reveal about a SDE candidate? It has nothing to do with computer science. If it's a real question, that interviewer should cease and desist being involved in interview loops.

Let's say the principle amount is m and interest rate is r% and is calculated annually. so after n years the amount will be m*(1+ r/100)^n.
Here m=1 and r = 100 so the amount will be 1*(1+100/100)^n = 2^n.
given that 2^n = 15B. so n = ln(15B)
ln means log with base 2

The question is incomplete. The answers would differ for a simple interest vs compound interest.

log2 (x) = logy (x) / logy (2) use built in log

n=15b;
while(n!=1)
{
n=n/2;
}

n=15b;
year=0;
while(n!=1)
{
year++;
n=n/2;
}