Question 540376: Alfreida doubled her money in the stock market every 4 years. After 20 years, she had 120,000 dollars. How much did she start with? Answer by jpg7n16(66) (Show Source):
You can put this solution on YOUR website! Since Alfreida doubled her money every 4 years, after 20 years she will have doubled her money 5 times.
So, we know she started with some amount of money $X, which doubled 5 times and became $120,000. That means her X was multiplied by 2 again and again - a total of 5 times. As an equation, that looks like this:
or (the much easier way) using exponents, the 2's are more easily read as:
and since 2^5 = 32, the formula then becomes
Then to solve for her original x, just divide each side by 32.
So we know Alfreida started with $3,750.
(