Answer:
(a) The value of fₓ (9.5) is 0.125.
(b) The value of fₓ (10.5) is 0.50.
Step-by-step explanation:
Let <em>X</em> denote delivery time of the mail delivered by Alice and <em>Y</em> denote delivery time of the mail delivered by Bob.
It i provided that:

The probability that Alice delivers the mail is, <em>p</em> = 1/4.
The probability that Bob delivers the mail is, <em>q</em> = 3/4.
The probability density function of a Uniform distribution with parameters [<em>a</em>, <em>b</em>] is:

The probability density function of the delivery time of Alice is:
![f(X_{A})=\left \{ {{\frac{1}{b-a}=\frac{1}{2};\ [a, b]=[9, 11]} \atop {0;\ otherwise}} \right.](https://tex.z-dn.net/?f=f%28X_%7BA%7D%29%3D%5Cleft%20%5C%7B%20%7B%7B%5Cfrac%7B1%7D%7Bb-a%7D%3D%5Cfrac%7B1%7D%7B2%7D%3B%5C%20%5Ba%2C%20b%5D%3D%5B9%2C%2011%5D%7D%20%5Catop%20%7B0%3B%5C%20otherwise%7D%7D%20%5Cright.)
The probability density function of the delivery time of Bob is:
![f(X_{B})=\left \{ {{\frac{1}{b-a}=\frac{1}{2};\ [a, b]=[10, 12]} \atop {0;\ otherwise}} \right.](https://tex.z-dn.net/?f=f%28X_%7BB%7D%29%3D%5Cleft%20%5C%7B%20%7B%7B%5Cfrac%7B1%7D%7Bb-a%7D%3D%5Cfrac%7B1%7D%7B2%7D%3B%5C%20%5Ba%2C%20b%5D%3D%5B10%2C%2012%5D%7D%20%5Catop%20%7B0%3B%5C%20otherwise%7D%7D%20%5Cright.)
(a)
Compute the value of fₓ (9.5) as follows:
For delivery time 9.5, only Alice can do the delivery because Bob delivers the mail in the time interval 10 to 12.
The value of fₓ (9.5) is:

Thus, the value of fₓ (9.5) is 0.125.
(b)
Compute the value of fₓ (10.5) as follows:
For delivery time 10.5, both Alice and Bob can do the delivery because Alice's delivery time is in the interval 9 to 11 and that of Bob's is in the time interval 10 to 12.
The value of fₓ (10.5) is:

Thus, the value of fₓ (10.5) is 0.50.