bondguy wrote:

Came across this question on this site itself and I am struggling to get the answer the way they describe it and why my approach is at fault

here is the question:

EXAMPLE 10 A three-person committee must be chosen from a group of 7 professors and 10 graduate students. If at least one of the people on the committee must be a professor, how many different groups of people could be chosen for the committee?

A. 70

B. 560

C. 630

D. 1,260

E. 1,980

My approach :

Since we need to have a professor for the 3-member committee , we can choose the first person as professor in 7 ways .

Rest two positions can be filled with remaining professors or students in (16 C 2 )

total number of combinations for filling 3 member committe -

first position with professor * rest with 2 from 16 mombers ( 6 professors + 10 students) is

7 * (16 C 2) = 7 * (16 * 15 )/2

= 840

but what is baffling is total number of combinations for choosing 3 members from 17 ( 7 professors + 10 students) without restriction of atleast one professor in the committie = 17 C 2 = 680

how can this combination be less than restricted combination

can someone explain me where my thought process took a wrong turn ..

Here is how I solved this problem...

There are three cases in which you have a professor that is part of the comitte:

1 prof, 2 students

2 prof, 1 students

3 prof, 0 students

Here are the total number of combinations I found for each case:

1 prof, 2 students: 7*10*9 = 630

2 prof, 1 students: 7*6*10 = 420

3 prof, 0 students: 7*6*5 = 210

So then I got 1260 total (630+420+210) which is D.