21. On the day his son was born, Sam invested a single amount of money that would grow to $16000 when his son was 21 years old. If he invested the initial amount at 5% compounded quarterly, using exponential equations or expressions find the initial amount to the nearest dollar. 22. Evaluate the series 1 + 5 + 25 + ... + 15625 23. The amount of a substance decreases by 10% every hour. If there was 123 grams at the end of the first hour, how much was there after 12 hours? 24. Starting with the exponential equation y = 2' describe the transformations in words to develop the graph of y = 2(1-1) + 4. Provide a sketch of the graph showing essential features