Another combination question

Nov 2014
7
0
Canada
Six letters are to be chosen from 25 letters (order irrelevant). In how many ways can the letters be chosen if D and E must be selected. Can do directly: 23C4 and know it's easier, but how to do indirectly? Why doesn't 25C6 - 23C6 not work?
 

mathman

Forum Staff
May 2007
6,932
774
On what basis do you propose the wrong answer?
 
Nov 2014
7
0
Canada
23C4 does not equal 25C6 - 23C6
 
Nov 2014
7
0
Canada
Sorry, didn't understand what was being asked. I figured out why it doesn't work. I was originally thinking that I could take all the cases and subtract the cases where D and E were not included, but I realize that the way I wrote it is flawed. Thank you for your time.