A committee of four is to be selected from 12 people. In how many different ways can this be done if either Mary or stefan must be on this committee but not both. (we have to use nCr notation and we can use cas calc

Question

cwrw238 · Answer

Mary on committee:-  = 11C3
Stefan :- 11C3

cwrw238 · Answer

total ways  = 2 * 11C3

anonymous · Answer

yeah then what?

cwrw238 · Answer

well use you calculator to work out 11C3
11C3 =  (11*10*9)  / (3*2*1)

cwrw238 · Answer

11C3  means the number of combinations of 3 taken from 11

anonymous · Answer

yes but thats both. the question is asking for one or the other. not both. the answers is 240

anonymous · Answer

adding those two gives 330 but this include having both mary and stefan doesnt it?

cwrw238 · Answer

Yes - my mistake
its 2 * 10C3  = 2 * 120 = 240

anonymous · Answer

ok seem right. but why are you picking 3 from 10

anonymous · Answer

oh nvm i get it now. CHeers