In an animal hospital, 10 units of a certain medicine were injected into a dog. After 40 minutes, only 5 units remained in the dog. Let f(t) be the amount of the medicine present after t minutes. At any time, the rate of change of f(t) is proportional to the value of f(t). Find the formula for f(t). The formula is f(t)= | (Use integers or decimals for any numbers in the equation. Round to three decimal places as needed.)