Question 1
A committee of 7 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** 4 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: 7
- Constraint: At least 4 men
Strategy: We sum the ways for all cases from exactly 4 men up to the maximum possible number of men (7).
Valid Cases (Men, Women) and Calculation:
(4 Men, 3 Women): C(9,4) × C(7,3) = 126 × 35 = 4410
(5 Men, 2 Women): C(9,5) × C(7,2) = 126 × 21 = 2646
(6 Men, 1 Women): C(9,6) × C(7,1) = 84 × 7 = 588
(7 Men, 0 Women): C(9,7) × C(7,0) = 36 × 1 = 36
Final Calculation (Sum Rule):
Total ways = (Ways with 4 men) + (Ways with 5 men) + ...
= 4410 + 2646 + 588 + 36
= 7680
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: 7
- Constraint: At least 4 men
Strategy: We sum the ways for all cases from exactly 4 men up to the maximum possible number of men (7).
Valid Cases (Men, Women) and Calculation:
(4 Men, 3 Women): C(9,4) × C(7,3) = 126 × 35 = 4410
(5 Men, 2 Women): C(9,5) × C(7,2) = 126 × 21 = 2646
(6 Men, 1 Women): C(9,6) × C(7,1) = 84 × 7 = 588
(7 Men, 0 Women): C(9,7) × C(7,0) = 36 × 1 = 36
Final Calculation (Sum Rule):
Total ways = (Ways with 4 men) + (Ways with 5 men) + ...
= 4410 + 2646 + 588 + 36
= 7680
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 $k
e j$).