Question 1099905
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Notice that the problem is different if you change "MUST" to "CAN":
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There are 10 students from whom 4 are going to be chosen to 
represent their club at a meeting. If, Sue, Mark, or John,
but only one of them <font size = 4><b><u>MUST</u></b></font> be chosen, in how many ways can 
the students be chosen to go to the meeting?
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That's (3 choose 1) AND (7 choose 3)

(3C1)(7C3) = (3)(35) = 105 ways.

However if the problem were:

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There are 10 students from whom 4 are going to be chosen to 
represent their club at a meeting. If, Sue, Mark, or John,
but only one of them <font size = 4><b><u>CAN</u></b></font> be chosen, in how many ways can 
the students be chosen to go to the meeting?
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That's [(3 choose 1) AND (7 choose 3)] OR [(3 choose 0) AND (7 choose 4)]

(3C1)(7C3) + (3C0)(7C4) = (3)(35) + (1)(35) = 105 + 35 = 140 ways.

Edwin</pre>