Question

A delegation of $4$ students is to be selected from a total $12$ students, in how many ways can the delegation be selected?
(i) If all the students are equally willing?
(ii) If two particular students have to be included in the delegation?
(iii) If two particular students do not wish to be together in the delegation?
(iv) If two particular students wish to be included together only in the delegation
(v) If two particular students refuse to be together, and two other particular students wish to be together only in the delegation?

Answer 1

$\left(i\right)$ Formation of delegation means selection of 4 out of 12.

Hence the number of ways ${}^{12}{C}_4=495$

$\left(ii\right)$ If two particular students are already selected. Here we need to select only 2 out of the remaining 10.

Hence the number of ways ${=}^{10}{C}_2=45$

$\left(iii\right)$ The number of ways in which both are selected $=45$

Hence, the number of ways in which the two are not included together $=495-45=450$

$\left(iv\right)$ There are two possible cases

(a) Either both are selected. In this case, the number of ways in which the selection can be made $=45$

(b) Or both are not selected. In this case all the four students are selected from the remaining ten students.

This can be done in ${}^{10}{C}_4=210ways$

Hence the total number of ways of selection $=45+2210=255$

$\left(v\right)$ We assume that student A and B wish to be selected together and student C and D do not wish to be together. Now there are following 6 cases.

(A,B,C) selected, (D) not selected

Number of ways of selection ${=}^8{C}_1=8$

(A,B,D) selected, (C) not selected

Number of ways of selection ${=}^8{C}_1=8$

(A,B) selected (C,D) not selected

Number of ways of selection ${=}^8{C}_2=28$

(C) selected (A,B,D) not selected

Number of ways of selection ${=}^8{C}_3=56$

(D) selected (A,B,C) not selected

Number of ways of selection ${=}^8{C}_3=56$

(A,B,C,D) not selected

Number of ways of selection ${=}^8{C}_4=70$

Hence total number of ways $8+8+28+56+56+70=226$