Think About the Process A student has a one-dollar bill, a five-dollar bill, and a ten-dollar bill in a box. The student selects a bill from the box 40 times and selects the one-dollar bill 21 times. What formula should be used to calculate the experimental probability? What is the experimental probability of selecting the one-dollar bill? What is the theoretical probability of selecting the one-dollar bill? The experimental probability of selecting the one dollar bill is [_] ​