Sari puts $100 in 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)$210
B)$300
C)$800
D)$1,010

Question

Quizplus · Accepted Answer

Using the rule of 70, we divide 70 by the interest rate (10% = 0.1) to find the number of years it takes for the investment to double: 
70/0.1 = 700 years.

Since Sari is investing for 21 years, we can estimate that her money will approximately double twice: 
$100 x 2 = $200 (after 10 years)
$200 x 2 = $400 (after 20 years)

At this point, we can estimate that she has slightly more than doubled her money again in the last year, bringing her total to approximately $800. Therefore, the best choice is C.

Sari Puts $100 in an Account with an Interest Rate