Q1 - 72, 24, 48
Q2 - 3
Q3 - 11
Q4 - Because Jack has 99$, he will bet his entire money on Sprinter B, and if B wins he'll have 1188$
Bill should bet on Sprinter B as well. more than 20$ it doesn't matter what will happen, Bill win win the 1,000,000$.
I don't know if it was the answer you were looking for, but it's just logic.
Because there is no way for Jack to win if he doesn't bet on Sprinter B. Bill knows that. So he knows that if he'll bet even 15-20$, he secure his win. 100%.