A firm has 4 senior partners and 6 junior partners. How many different groups F of 3 partners can be formed in which at least one member of the group is a senior partner. (2 groups are considered different if at least one group member is different)  Choices.
A) 48  B) 100  C) 120  D) 288  E) 600
What seems to work as well, though its tougher to come up with this solution:     (10*9*8) * 2/3 + (10*9*4)*1/3= 
<gadget id="calculator">  (10*9*8) * 2/3 + (10*9*4)*1/3</gadget>
<output>600</output>
600. Divided by all possible permutations (=3!) yields 100.   Explanation: For the first spot you have 10 possible canditates, for the 2nd 9. For the 3rd spot you need to differentiate if a senior partner has been picked for one of the first 2 spots or not. If yes, then you can pick one of the 8 remaining guys, so 10*9*8. If no senior partner has been picked yet, you need 10*9*4.   Now you need to weight the 2 cases with the corresponding probabilities: probability of no senior partner being picked in the first 2 draws = 6/10*5/9=1/3, so you weight 10*9*4 with 1/3. For the complementary case (senior partner was picked in the first 2 draws) you just take the complementary prob (1-1/3)= 2/3 and weight 10*9*8 with it.   Now you just need to divide the answer (600) by the number of different positions (=3!) and get F=600/6=100.100
<result>B</result>
A firm has 4 senior partners and 6 junior partners. How many different groups F of 3 partners can be formed in which at least one member of the group is a senior partner. (2 groups are considered different if at least one group member is different)