Question 1
A committee of 8 members is to be formed from 9 men and 7 women. In how many ways can this be done if the committee must have **at least** 6 men?
Step-by-Step Solution (Sum Rule):
Concept: 'At least' problems require finding the sum of ways for all valid, mutually exclusive cases.
Given:
- Men available: 9
- Women available: 7
- Committee size: 8
- Constraint: At least 6 men
Strategy: We sum the ways for all cases from exactly 6 men up to the maximum possible number of men (8).
Valid Cases (Men, Women) and Calculation:
(6 Men, 2 Women): C(9,6) × C(7,2) = 84 × 21 = 1764
(7 Men, 1 Women): C(9,7) × C(7,1) = 36 × 7 = 252
(8 Men, 0 Women): C(9,8) × C(7,0) = 9 × 1 = 9
Final Calculation (Sum Rule):
Total ways = (Ways with 6 men) + (Ways with 7 men) + ...
= 1764 + 252 + 9
= 2025
Key Principle: Use the Sum Rule (addition) because the cases are mutually exclusive (you cannot simultaneously select exactly $k$ men and exactly $j$ men, where $k
e j$).
Concept: 'At least' problems require finding the sum of ways for all valid, mutually exclusive cases.
Given:
- Men available: 9
- Women available: 7
- Committee size: 8
- Constraint: At least 6 men
Strategy: We sum the ways for all cases from exactly 6 men up to the maximum possible number of men (8).
Valid Cases (Men, Women) and Calculation:
(6 Men, 2 Women): C(9,6) × C(7,2) = 84 × 21 = 1764
(7 Men, 1 Women): C(9,7) × C(7,1) = 36 × 7 = 252
(8 Men, 0 Women): C(9,8) × C(7,0) = 9 × 1 = 9
Final Calculation (Sum Rule):
Total ways = (Ways with 6 men) + (Ways with 7 men) + ...
= 1764 + 252 + 9
= 2025
Key Principle: Use the Sum Rule (addition) because the cases are mutually exclusive (you cannot simultaneously select exactly $k$ men and exactly $j$ men, where $k
e j$).