Answer:
The probability that the test is taken an even number of times is 0.30.
Step-by-step explanation:
The probability that a student passes the driving test at any attempt is,
<em>p</em> = 4/7.
The event of a student passing in any attempt is independent of each other.
The probability that the test is taken an even number of times is:
P (even number of tests) = P (Passing in the 2nd attempt)
+ P (Passing in the 4th attempt)
+ P (Passing in the 6th attempt) ...
If a student passed in the 2nd attempt it implies that he failed in the first.
Then, P (Passing in the 2nd attempt) = ![(\frac{3}{7}) \times (\frac{4}{7})](https://tex.z-dn.net/?f=%28%5Cfrac%7B3%7D%7B7%7D%29%20%5Ctimes%20%28%5Cfrac%7B4%7D%7B7%7D%29)
Similarly, P (Passing in the 4th attempt) =
, since he failed in the first 3 attempts.
And so on.
Compute the probability of an even number of tests as follows:
P (even number of tests) = ![(\frac{3}{7}) \times (\frac{4}{7})+(\frac{3}{7})^{3} \times (\frac{4}{7})+(\frac{3}{7})^{5} \times (\frac{4}{7})+...](https://tex.z-dn.net/?f=%28%5Cfrac%7B3%7D%7B7%7D%29%20%5Ctimes%20%28%5Cfrac%7B4%7D%7B7%7D%29%2B%28%5Cfrac%7B3%7D%7B7%7D%29%5E%7B3%7D%20%5Ctimes%20%28%5Cfrac%7B4%7D%7B7%7D%29%2B%28%5Cfrac%7B3%7D%7B7%7D%29%5E%7B5%7D%20%5Ctimes%20%28%5Cfrac%7B4%7D%7B7%7D%29%2B...)
The result follows a Geometric progression for infinite values.
The sum of infinite GP is:
![S=\frac{a}{1-r^{2}}](https://tex.z-dn.net/?f=S%3D%5Cfrac%7Ba%7D%7B1-r%5E%7B2%7D%7D)
The probability is:
P (even number of tests) = ![(\frac{3}{7}) \times (\frac{4}{7})+(\frac{3}{7})^{3} \times (\frac{4}{7})+(\frac{3}{7})^{5} \times (\frac{4}{7})+...](https://tex.z-dn.net/?f=%28%5Cfrac%7B3%7D%7B7%7D%29%20%5Ctimes%20%28%5Cfrac%7B4%7D%7B7%7D%29%2B%28%5Cfrac%7B3%7D%7B7%7D%29%5E%7B3%7D%20%5Ctimes%20%28%5Cfrac%7B4%7D%7B7%7D%29%2B%28%5Cfrac%7B3%7D%7B7%7D%29%5E%7B5%7D%20%5Ctimes%20%28%5Cfrac%7B4%7D%7B7%7D%29%2B...)
![=\frac{(\frac{3}{7})(\frac{4}{7} ) }{1-(\frac{3}{7})^{2}}\\=\frac{12}{49}\times\frac{49}{40}\\ =\frac{12}{40}\\ =0.30](https://tex.z-dn.net/?f=%3D%5Cfrac%7B%28%5Cfrac%7B3%7D%7B7%7D%29%28%5Cfrac%7B4%7D%7B7%7D%20%29%20%7D%7B1-%28%5Cfrac%7B3%7D%7B7%7D%29%5E%7B2%7D%7D%5C%5C%3D%5Cfrac%7B12%7D%7B49%7D%5Ctimes%5Cfrac%7B49%7D%7B40%7D%5C%5C%20%20%3D%5Cfrac%7B12%7D%7B40%7D%5C%5C%20%3D0.30)
Thus, the probability that the test is taken an even number of times is 0.30.