Question

We wish to select 6 persons from 8, but if the person A is chosen, then B must be chosen. In how many ways can the selection be made?

Answer 1

6 people are to be selected from 8.

There are two case.
(i) When A is selected, then B must be chosen.
∴ $\mathrm{Number}\mathrm{of}\mathrm{ways}={}^6{C}_4=15$

(ii) When A is not chosen:
$\mathrm{Number}\mathrm{of}\mathrm{ways}{=}^7{C}_6=7$

∴ Total number of ways = 15 + 7 = 22