Question 247538
A box contains 50 razor blades, five of these are known to be used, the remainder unused. What is the probability that exactly 3 will be used.
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The numerator is the number of successful ways, which is the
number of ways to select exactly 3 from the 5 used.
That's "5 choose 3"

or 5C3 = {{{(5*4*3)/(3*2*1)= 60/6=10}}}

The denominator is the number of possible ways, which is the
number of ways to select exactly 3 from the total 50.
That's "50 choose 3"

or 50C3 = {{{(50*49*48)/(3*2*1)= 117600/6=19600}}}

So the desired probability is

{{{10/19600=1/1960}}}

Edwin</pre>