The possible values of N from the given probability parameters is; N equals 15 or 21
We are told that;
Number of pens in the box = N
Let the number of red pens be x
Thus; number of black pens = x + 3
Thus;
x + x + 3 = N
x = (N - 3)/2
The probability that she will take a black pen followed by a red pen is;
P(she will take a black pen followed by a red pen) = (x + 3)/N * x/(N - 1)
Plugging in he relevant values and solving for N gives ;
Solving for N gives N equals 15 or 21
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