Question 1
A committee of 6 members is to be formed from 8 men and 7 women. In how many ways can this be done if the committee must have **at least** 3 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: 8
- Women available: 7
- Committee size: 6
- Constraint: At least 3 men
Strategy: We sum the ways for all cases from exactly 3 men up to the maximum possible number of men (6).
Valid Cases (Men, Women) and Calculation:
(3 Men, 3 Women): C(8,3) × C(7,3) = 56 × 35 = 1960
(4 Men, 2 Women): C(8,4) × C(7,2) = 70 × 21 = 1470
(5 Men, 1 Women): C(8,5) × C(7,1) = 56 × 7 = 392
(6 Men, 0 Women): C(8,6) × C(7,0) = 28 × 1 = 28
Final Calculation (Sum Rule):
Total ways = (Ways with 3 men) + (Ways with 4 men) + ...
= 1960 + 1470 + 392 + 28
= 3850
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: 8
- Women available: 7
- Committee size: 6
- Constraint: At least 3 men
Strategy: We sum the ways for all cases from exactly 3 men up to the maximum possible number of men (6).
Valid Cases (Men, Women) and Calculation:
(3 Men, 3 Women): C(8,3) × C(7,3) = 56 × 35 = 1960
(4 Men, 2 Women): C(8,4) × C(7,2) = 70 × 21 = 1470
(5 Men, 1 Women): C(8,5) × C(7,1) = 56 × 7 = 392
(6 Men, 0 Women): C(8,6) × C(7,0) = 28 × 1 = 28
Final Calculation (Sum Rule):
Total ways = (Ways with 3 men) + (Ways with 4 men) + ...
= 1960 + 1470 + 392 + 28
= 3850
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 $k
e j$).