Question

Exercise 5.6.6: Selecting a committee of senators. About A country has two political parties, the Demonstrators and the Repudiators. Suppose that the national senate consists of 100 members, 44 of which are Demonstrators and 56 of which are Rupudiators. (a) How many ways are there to select a committee of 10 senate members with the same number of Demonstrators and Repudiators? Suppose that each party must select a speaker and a vice speaker. How many ways are there for the two speakers and two vice speakers to be selected? Feedback?

Answer 1

There are 4,148,350,734,528 ways

Explanation:

We have

• 44 senators which are Demonstrators.
• 56 senators which are Repudiators.

(a) How many ways are there to select a committee of 10 senate members with the same number of Demonstrators and Repudiators?

We want to choose 5 Demonstrators and 5 Repudiators. The number of ways to do this is
`{44} \choose {5}` and
`56 \choose 5` respectively. Therefore, the number of ways to select the committee is given by:

`{{44}\choose {5}} * {{56}\choose{5}}=(44!)/(39!5!)*(56!)/(51!5!)=(44!56!)/(51!39!5!5!)=(44*43*42*41*40*56*55*54*53*52)/(5!5!)=\\\\=(44*43*42*41*8*56*11*54*53*52)/(4!4!)= (11*43*42*41*2*56*11*54*53*52)/(3!3!)=\\\\(11*43*14*41*2*56*11*18*53*52)/(2!2!)=`

`11*43*14*41*28*11*18*53*52=4,148,350,734,528`

(b) Suppose that each party must select a speaker and a vice speaker. How many ways are there for the two speakers and two vice speakers to be selected?

• If the speaker and vice speaker are chosen between all senators: In this case, the answer will be

`44*43*56*55=5,827,360.`

This is because there are (in the case of Demonstrators) 44 possibilities to choose an speaker and after choosing one, there would be 43 possibilities to choose a vice speaker. The same situation happens in the case of Repudiators.

• If the speaker and vice speaker are chosen between the committee: In this case, the answer will be

`5*4*5*4=400`.