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Math Help - pmf and support

  1. #1
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    pmf and support

    In a smoking survey among boys between the ages of 12 and 17, 28% prefer to date nonsmokers, 1% prefer to date smokers and 21% don't care. Suppose 7 such boys are selected randomly and X equals the number who prefer to date nonsmokers and Y equals the number who prefers to date smokers.

    A. Determine the joint p.m.f. of X and Y. Be sure to include the support of the p.m.f.
    B. Find the marinal p.m.f. of X. Again include the support.

    The support for A. I figured was O \le x \le 7, 0 \le y \le 7, 0 \le x+y \le 7. The p.m.f. would be
    combinations(X of 7) *combinations(Y of 7-X) *(0.78)^{x}*0.01^{y}*0.21^{(7-x-y)}

    I can't figure out what values to use for x and y.

    The support I put for B. is 0 \le x \le 7<br />
with p.m.f.=comb(X of 7) *(0.78)^{x}*(0.22)^{(7-x)}

    I've been working on this problem, but I'm still really confused about how to perform the equations. Thank you for your help.
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  2. #2
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    Quote Originally Posted by logitech View Post
    In a smoking survey among boys between the ages of 12 and 17, 28% prefer to date nonsmokers, 1% prefer to date smokers and 21% don't care. [snip]
    What about the other 50% ....?
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  3. #3
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    I'm sorry. The 28% was supposed to be 78% prefer to date nonsmokers. I'm not sure how the combination parts should be written to show the proper formula. Any help would be great!
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