Granny puts $35,000 into a bank account earning 4%. You can't withdraw the money until the balance has doubled. How long will you have to leave the money in the account

It will take 17 years and 245 days.

Explanation:

Giving the following information:

Granny puts $35,000 into a bank account earning 4%. You can't withdraw the money until the balance has doubled.

We need to find the number of years required to achieve $70,000.

We will use a variation of the final value formula:

FV = PV * (1+i) ^n

Isolating n:

n=[ln (FV/PV) ]/ln (1+r)

n = [ln (70,000/35,000) ] / ln (1.04) = 17.67

To be more accurate:

0.67*365 = 245 days

It will take 17 years and 245 days.