This is a binomial probability. The probability of getting r successes out of n trials where the probability of success each trial is p and probability of failure each trial is q (where q = 1-p) is given by:
r (n-r)
n!(p )[q ]/r!(n-r)!. Here n = 20, p = 0.5 and q = 0.5 and r = 15,16,17,18,19,20. Therefore we have
15 5
P(15) = 20!(0.5 )(0.5 )/15!5! = 0.0148
16 4
P(16) = 20!(0.5 )(0.5 )/16!4! = 0.0046 P(17) = 0.0011 P(18) = 0.0002 P(19) = 0.00002 P(20) = 0.000001
The sum adds up to 0.0207.

Since a price decline led to an increase in total expenditures on air travel, the demand for air travel must be elastic. This is because elastic demand implies that a 10 percent decline in price will lead to a greater than 10 percent increase in demand. Thus, under elastic demand and a price decline, total expenditures are expected to rise.

The number of days in the accounts payable period: 46 days. The number of days in the accounts receivable period: 57 days. The number of days in the cash cycle: 72 days.