1. 20 years ago, David opened a savings (investment) account and made a deposit for an amount you don't remember. After having made mentioned deposit, David has kept his money in his account and you have not made another deposit. It is known that the savings account paid a interest rate of 20% per year for the first 10 years and 15% during the remaining years. The current account statement indicates that David he has $7,514,708,154 in the bank. How much did David deposit 20 years in the bank?

2. How long does it take for an amount deposited in a savings account to be doubles if the bank offers a yield (an interest rate) of 20% annual?