Sari puts $100 into an account with an interest rate of 10 percent. According to the rule of 70, about how much does she have at the end of 21 years?

a. $800

b. $210

c. $1,010

d. $300

The correct answer is A: $800

Explanation:

The rule of 70 is a means of estimating the number of years it takes for an investment or your money to double.

Number of Years to Double = 70/Annual Rate of Return

In this exercise = 70/10 = 7 years

I=100

i=0,10

Every 7 years the investment duplicates.

7 years = 200

14 years = 400

21 years = 800