21. For the stock broker problem described in Problem 16 in Chapter 9 , assume that among the qualified clients, half will invest between $2,000 and $10,000, 25% will invest between $10,000 and $25,000, 15% will invest between

$25,000 and $50,000, and the remainder will invest between $50,000 and $100,000, each uniformly distributed.

Using the same commission schedule, how many calls per month must the broker make each month to have at least a 75% chance of making at least $5,000?