The prime numbers that lie between 1 and 21 are 2,3,5,7,11,13,17, and 19. These 8 balls, plus the 10-ball comes to a total of 9 balls that could be picked out of the 21. Therefore, the probability of picking one of these balls is 9/21 or 3/7
Let's first convert that amount of money into cents, which makes 336 cents. Then, let's denote the amount of money one of the boys has with x. The other boy has got 3x-12 amount of money. We can construct an equation. . Solving it we find that x=87 cents or $ 0.87 and the other boy holds $2.49 amount of money.