## katiebugg 3 years ago I am going to choose 6 students out of the 15 who currently have A's to be mentors for the students who have C's or lower. How many ways can a group of 6 be selected?

1. alexwee123

|dw:1343977571380:dw|

2. katiebugg

i have no idea

3. alexwee123

this might help :) |dw:1343977665844:dw|

4. katiebugg

i think so how would i aply that

5. alexwee123

for your question n=15 and r=6 so |dw:1343977816649:dw| now can you solve it?

6. katiebugg

why the !

7. alexwee123

oh it means factorial take "5!" for example 5!=5x4x3x2x1

8. alexwee123

so apply this to the current problem :o

9. katiebugg

soo whats 15 c x 10

10. alexwee123

what do you think it is?

11. katiebugg

idk am i multiplying it by the 15

12. alexwee123

13. alexwee123

but do you get how i got this?

14. katiebugg

i want to say yess but i cant

15. alexwee123

ok look :o $15!=15\times14\times13\times12\times11\times......\times1$ and$9!=\times8\times7\times6\times5\times...1 and 6!=6\times5\times4\times3\times2\times1$ so if you sub this in, we can easily simplify the equation $(15\times14\times13\times12\times11\times10\times9!)\div(6!(9!))$ you can cancel out the 9! and be left w/$(15\times14\times13\times12\times11\times10)\div(6!)$ then just simplify by using intuition

16. katiebugg

ohhh ok lol