Question 1
A committee of 6 members is to be formed from 10 men and 6 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: 10
- Women available: 6
- 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(10,3) × C(6,3) = 120 × 20 = 2400
(4 Men, 2 Women): C(10,4) × C(6,2) = 210 × 15 = 3150
(5 Men, 1 Women): C(10,5) × C(6,1) = 252 × 6 = 1512
(6 Men, 0 Women): C(10,6) × C(6,0) = 210 × 1 = 210
Final Calculation (Sum Rule):
Total ways = (Ways with 3 men) + (Ways with 4 men) + ...
= 2400 + 3150 + 1512 + 210
= 7272
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: 10
- Women available: 6
- 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(10,3) × C(6,3) = 120 × 20 = 2400
(4 Men, 2 Women): C(10,4) × C(6,2) = 210 × 15 = 3150
(5 Men, 1 Women): C(10,5) × C(6,1) = 252 × 6 = 1512
(6 Men, 0 Women): C(10,6) × C(6,0) = 210 × 1 = 210
Final Calculation (Sum Rule):
Total ways = (Ways with 3 men) + (Ways with 4 men) + ...
= 2400 + 3150 + 1512 + 210
= 7272
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 $k
e j$).