Element X is a radioactive isotope such that every 18 years, its mass decreases by half. Given that the initial mass of a sample of Element X is 500 grams, how long would it be until the mass of the sample reached 210 grams, to the nearest tenth of a year?

At the beginning of year 1, carlos invests $600 at an annual compound interest rate of 4%. he makes no deposits to or withdrawals from the account. which explicit formula can be used to find the account's balance at the beginning of year 5? what is, the balance?