Question

You are looking for an account to invest your $9,000 in.  You want to know how many years it will take to double if the account you are putting it into gets 10% APR.  Using the Rule of 70, how many years should you be expecting to leave it sit?

7 years

10 years

18 years

You take $9,000 and place it into an account that compounds quarterly (4 times a year) at 7% for 10 years.  What is the account going to be worth at the end of that time?  Round to the nearest penny.

$16,201.83

$15,300.00

$18,014.38

Answer 1

I thought it was the rule of 72.
Anyway by the "rule of 70" it would clearly take 7 years.
You multiply the time by the rate

actual time:
2 = 1.1^n
log2 = nlog 1.1
n = log2/log1.1 = 7.27

the rule of 72 would have given an answer of 72/10 = 7.2 , a bit closer to the real answer.

#2, amount = 9000(1.0175)^40 = .....

I see the correct answer in your list